The level of jealousy you feel probably indicates something in your past that might be usefully addressed in therapy.  There may be things from your past feeding into the current situation. 

Agree with Kes.

Or, there is the possibility that you aren't compatible. Wanting to share big "firsts" with someone means more to some people than others. For me, it's not a deal breaker. For you, it might be. Wanting to have someone's first kid is no different than any other "thing" that you want out of a relationship - whether that be to travel the world, maintain a sizeable savings account, always live near family, never want to have kids, etc.

Try therapy and see if you can overcome the jealousy. If you can't, you may have to come to terms with the fact that either this relationship or any relationship with a person with kids just isn't right for you. And there isn't anything wrong with that.

Try and view the positives. First time someone becomes a parent it can be a stressfull experience, and one can often see with the benefit of hindsight how to deal with a situation better. 

It is possible he may make a more relaxed father second time around if you were to have any together, this is an extremely valuable quality that some people (but not all) have. 

There are a lot of things I did before meeting my husband that I enjoy just as much with him. Before having a child I struggled with these same feelings. 

This might seem silly but there is a festival I would go to every year with my friends, boyfriend and brother. I always had fun no matter who I was with. When I took my DH the first time it was no less fun because I had gone previously with a different boyfriend. 

with my bf or anyone. At first it was really hard for me too because I never envisioned for myself that I would be with someone who has already been married and had children before. I wanted to experience the firsts of that with someone where it was also their first. However, my bf has made me feel a lot better about my insecurity and jealousy around the topic. He explained to me how it would not be his first, but it would be his first with me and that would be extremely special. Plus his children were not planned so it would be a first time trying and planning for a baby.

Personally, I would also like to give him a son, that would be a first for us both as he has two daughters, but does not have a son. There are ways that you can make firsts with him still on the subject and even still it will be completely different with you than it was with his ex.

I guess my best advice is for you to seek therapy because I think that this jealousy has to be rooted in some level of insecurity on your part.. that you aren't "enough".

Look,  I will say that I have REGRET that I didn't meet my DH earlier in his life.  I wish I had 100 lifetimes to spend with him.. and we met in our late thirties.. so we missed time together.  I missed being with him when he was in his teens.. twenties et.. but I am NOT jealous of his EX wives(two of them..lol).  I'm not jealous that he had kids before he met me with one of them.. I am regretful of the TIME I missed with him.

So, when I see old pictures of him.. it's a wistful feeling of wishing I had known him then.

I'm not even jealous that he had kids with his EX.  I mean, I am not so naive as to not realize that he had a life before we met.  That doesn't mean that our wedding and our hopes and dreams are any less tarnished because he may have had them with his EX.  Now, I wish there wasn't an EX because it complicated things.  I even wonder what it would have been like if I had had kids with him.  But.. it's not a jealousy emotion.. 

Because,  I 100% know that he and I are the right people to be together.  Whatever he did or wanted with his EX is done.. over.. and pales in comparison to the TRUE relationship that we have together.  I mean, I don't even think about his EX really any more now that his girls are adult age and we don't have to deal with that harpy screeching over her CS.

Your SO wants to be with you... he doesn't want his EX.  He may love his kids he had with her.. but he will love any child he has with you to the maximum as well.  It will be just as wonderful to have that as his first child.  Now, he may be a little less nervous about some things since he has some experience.. but the anticipation.. the love he will have for you and the baby.. it's not 2nd best.. at all.  If he makes you feel in any way that you aren't his best.. then maybe he is not the right one for you.

I would also get this straight in your own head because it's going to ruin your relationship to punish him for his past. (unless he was a serial rapist or an axe murderer.. hold that against a guy..haha).

Honestly, I used to feel this way but I don’t anymore. I think for me time and being around the BM/skid dynamic erased any and all jealousy I ever had of BM. She is no one to be jealous of, as she’s a self centred, manipulative, vapid human being, and there’s something wrong with me if I ever feel jealous of a person like that. Mostly I look at DH now incredulously that he ever found her attractive enough to reproduce with, let alone have any sort of relationship with. Men are stupid & think with their penises a lot of the time though. 

If it’s really important you have those firsts with someone though I’d recommend dating a man who doesn’t have kids. Or changing the narrative in your head so that his past doesn’t take up so much real estate in your mind, you’re only poisoning yourself by allowing that to happen. 

I really love my husband in many different ways. But I realized when I met him that if he had not had his relationship with his ex and there was no child, he would not have been the man I fell in love with and I probably wouldn’t have even met him as having a kid changed him for the better, and it changed the course of his life. 

Things would be a lot easier without his ex and a step kid but that is not reality. 

Why would I be jealous of a woman whose relationship broke down with the father of her child?

Jealousy is a normal emotion in step situations. You share your partner with others in ways that most of us never imagined and never experienced before. 

Communication is the best way to get through these issues, and is vital in healthy relationships. Talk to your partner about your feelings and fears. Sometimes hearing their point of view can help. Sometimes there are small adjustments that they can make to reassure you that you are number 1 for them. 

Lamenting the past, yours or anyone else's, is a waste of time.  There is nothing you can do about it.

So, is this a deal breaker or not?

That is the only question.

Think with your brain and not your feelings and emotion.  Feelings and emotion are not effective desisioning tools. Emotional intelligence is an important augmentation to decisioning.  But it is not a stand alone characteristic if a person has it.

If this guy is THE one quit sabotaging the relationship.  If you can't manage to do that then do him a favor and move on.

Maybe you just need to take things slowly and give the relationship time to mature. If love is real between you this won't seem like a big deal the longer you are together. I am my husband's third wife. One was high school marriage due to pregnancy, the second lasted over 20 years and 2 more kids. I could be and am jealous sometimes wondering if the second wife was the love of his life. (She cheated on him and treated him terribly finally causing him to divorce her but I know he didn't want too). On the other hand, I also know he is crazy about me and tells me how he wishes we could have had a child together and he had met me first. So, I just don't think about her and we live our life together and are thankful for the present.  Give yourself some time. If it really eats at you then move on because you will probably always have to deal with her, even when the child is an adult.

I felt very much the same way about my DH, who had 2 DSs when we married. However, I finally decided that I loved my DH and wanted to have a baby with him, so we planned it to the nth degree! (Even tried to make it a girl, which failed miserably! ha ha ha!) My pregnancy and the birth of our first son together was OUR first time, and it truly seemed every bit as special. The bonus was, DH, who has always been a hands on dad, knew EXACTLY how to handle a newborn, gave him his first bath, etc, when I was terrified. One down side I think I should mention is, when the subject of my pregnancy or giving birth would come up around DH's family, they could not help mentioning when SKs were born, (Youngest SS had to be life flighted to another hospital, so it was a big event.) Even after 20+ years with DH, I HATE hearing those stories, even though I'm sure DH's family thinks nothing of it and doesn't do it to be malicious. I just don't think people realize how hurtful those comments can be unless they've been in the situation, and you can't control what other people say, just how you react. Just another thing to keep in mind that you might have to prepare yourself for and work through if you continue in this relationship.

The feelings you have are valid and telling you that you're in a subpar situation. It sounds like this relationship is wrong for you on so many levels and you know it.  You don't need therapy or time to see if you can "get over" these feelings.  If you scroll through singles sites you'll see profiles that say, "No Divorcee's."  They're the smart ones who know the extent of the baggage they want to deal with.  Be like them!