Yes forever is exactly that...forever. But we don't have to try and figure out forever right now this minute. Or ever.

Take life a day at a time. Practice being in the moment and enjoying it.

There was so much chaos before in my life that I lost joy in anything. Now I 'try' to remember to 'enjoy' every thing, even the little things, as much as possible. It takes practice but it does help me to stay focused and be positive. Helps calm my mind too. I don't have to try and be a fortune teller predicting what may or may not happen in the future. And it helps me not get bogged down so much in all the what ifs. The serenity prayer is good but it doesn't always quite do the trick for me either. Mindfullness has helped me a lot.

When I showered this morning I thought about how good the water felt. How good my lavender body wash smelled. How clean and soft my skin felt after. It's very relaxing and calming to do that. Gives the mind a break from the crap and lets a little positivity in.

So your partner had 'firsts' with someone else. So what??? You and your partner will have your own firsts together. Everything you and your partner do the first time is YOUR first as a couple. You have had other holidays, trips, or whatever. Your partner has had other holidays, other trips, etc. But now you two do them together. Build your own memories. 

Do set good boundaries and communicate things with your partner. Boundaries and feelings can change over time...and that's ok. They shouldn't necessarily be stagnant but always be true to yourself. 

Yes there's an ex and kids involved. You and your partner need to be on the same page about things. This stuff and everything else.

Communicate, communicate, communicate!!!

The ex is just that...the ex. You and your partner are the 'here and now'. 

Hang in there. Navigating all this can be a wild ride but take care of you, and did i mention communicate things with your partner :)  And enjoy today!!

Once a breeding couple splits the blanket there really is zero reason for them to have any interface beyond transferring their children.  Particularly when an X is a toxic manipulative PASing evil POS and the quality side of that failed prior family recouples with a great new partner.

As much as one or the other of the failed prior family mates may want a controlling connection, the only way that can happen is if that persons X tolerates it.

I for one would not tolerate my mate having anything to do with their X beyond the bare minimum required to safely transfer a kid and would not tolerate one cent of support beyond what is COd.

My marital resources would not go to benefit an X. Regardless of which earner in my marriage earned that cent.

I would sit your DH down for a pointed discussion if I were you.

As for his firsts with an X.  Purge that from your head.  All his X has of your mate is his past. You are his present and his future.  Together you will experience an ongoing series of firsts that this X will not have with him.  Even if there are things he and is X first shared together, when you and he do those things together, it will still be the first time you have done those things together and the X's claim to that first is trumped and mitigated.

I have never lamented my DW's past.  Nor have I lamented my own.  Our pasts are what in large part makes us the person that our mates love, and what makes them the person that we love.  I would not change a thing.

Get out of our own head, out of your own way, and out of the way of being happy.

IMHO of course.

As some others said, the BM part can be handled with good boundaries. 
 

The other stuff about the "firsts" is YOUR issue. If that's important to you, then you should find a man with no kids who has never been married before. It's an unchangeable reality and maybe your feelings about it is telling you he's not the right guy. For some people, that stuff is more important than for others, and if you find that you can't get past it, then let this guy go. 

I used to be jealous of BM because she had all the firsts with my DH. However, she blew it all up, the house on the hill, the marriage to a hard working man and her two kids all because she couldn't keep her legs closed outside her marriage. She was with DH for 8 years. I've been with him for more than triple that time. Your SO's ex is is past and you're his present and forever. I've been unhappy and my H got us into counseling because he wants to make me happy and because he's very proud that we've been together for so long. I would take that over firsts any day.