I don't know for sure but I'd imagine that some of the reason that your DD isn't "believing" you is that she is being change resistant.  She's feeling insecure about all the changes in her life and is looking back at the past as a perfect time to back up the narrative she has going on in her brain.  Its a very human thing to do.  Its the number one reason organisational changes fail in the corporate world.  Its also the reason that when I meet up with college friends we all remember the time as amazing. - parts were but not the full 4 years.  We just skip past the bad bits.

I don't know what to recommend to try to fix this but there are some clever folks on here who will have some suggestions.

I agree with accidentalSM's assessment.. but further on I would add this.

Your daughter has experienced a lot of change in the past few years.. YOU have too!  Your relationship is fairly new at this point isn't it?  Less than a year?  And you both have teens that are within a year or two of graduating. 

While I think it is important to continue to encourage friendly contact with the two girls... I do think that you should consider delaying marriage/living together until they are both graduated from HS and off to college/living away from home.

In the grand scheme of things, it's not a super long time in relation to the relatively short time you have been together.  It will give you both time to enjoy some independent nurturing of your kids as they blossom to independance.. it also gives you time to truly know each other better before entwining yourselves with a joint home.. finances..etc.. You already lost a significant amount in your last marriage.. no need to rush in right?

Perhaps without the forced closeness of living together.. the two girls will be more likely to slowly warm to each other? 

I'm not saying to do this because the girls may want it.. but because it's the smart thing to do.

Yes.. I know it might save a bit of change combining households.. but that could also be lost if you have to quickly dissolve the household if things become problematic.  Plus.. with both girls away.. you would be able to potentially find a home that is more suited to your smaller full time family unit? maybe even not in the same area?

It's not like you are likely looking to have more kids I'm guessing.. so while I understand the desire for comittment.. I think that prudent delay of cohabitation will end up making everyone's lives a lot more calm and drama free.

As for your daughter's alleigances.. maybe over time and with more of her own life experience.. she will understand that not everything is visible.. in fact you hid the bad stuff from her to not expose her to the negativity.. I don't think you need to go overboard in trying to vindicate yourself though.. a simple.. "no one knows what is happening behind closed doors.. while I'm probably not totally blameless... I certainly by no means bear all the blame for the breakup.. sometimes relationships are toxic and you have to get out".

Even if she has "amnesia" and doesn't want to believe what happened, it's entirely possible that her body DOES know what happened and is experiencing some amount of trauma from it even if her brain protects her from it.

And it's not fair to expect her to build relationships with people she didn't choose to be part of her life. Expect civility, sure. But relationships are two-way streets, and she's saying she doesn't want one. That's her right.

And, since your last relationship was abusive and toxic, she's going to be hesistant of anyone new coming into your or her life. She's 17, so she probably can't fully process everything that happened AND her feelings around the situation. She's probably asking herself what work you've done to not find another d-bag whose going to treat you and her poorly.

To put it bluntly, she doesn't trust your judgment and the only thing that may get her to come around is time. Either you ex was that bad and she has to grapple with not trusting you OR your ex wasn't that bad so she has to grapple with you being a liar.

Being with a new guy may have helped you heal or feel better, but your BD didn't get a happy new relationship to help soothe her out of this. In fact, she LOST a relationship with someone who she doesn't fully know why he's gone (or at least doesn't want to face the truth about it). 

You two are experiencing this change very differently, and I don't think how she is expressing that is wrong. Your BD likely needs therapy to help process her feelings, not just another 17 year old who is going to agree with her friend. You can't convince her differently, either, because you're the unreliable narrator in this story. That absolutely sucks for you and is another wound you have to deal with thanks to your ex. But you can't move on and drag your DD into the future without meeting her where she is at, too. 

I'm sure this will be wildly unpopular with the OP but I'm going to agree that it is early days in your new relationship to be moving in with your boyfriend. You have a child who is nearing college age yet she is still your primary responsibility. Sometimes, as parents, we need to subvert our wants for the needs of our children. If you proceed with your plans to move in with your new boyfriend, you'll be sending a message to your daughter that the man in your life and your desire to live with him are more important to you than she is.

Take it from me. I put a man and my desire to be with him above my children at one point in my life. It took years before my daughter and I were close again.

She's leaving for college this August - 3 months from now?  I'd wait.  These are your last few months with her before she leaves.  If she's uncomfortable moving in with BF and his daughter, I don't see the harm in waiting a few months.  Coming home for breaks is going to be different whether your boyfriend is then part of the household or not - I don't think you need to "prepare" her for that.  Most kids I've known who go away to college consider school to be home and your home to be a temporary place to crash during breaks.  Is she asking you to put off the move for more than a few months?

DH and I didn't move in together until both our youngest were off to college. In fact we didn't move in together until a year after we were married so that both kids could finish high school in "their" places. During college, both kids came back to our new house during breaks, and it was a nice transition to "you're an adult" now. They each had their own rooms with their own things, and to my knowledge neither were uncomfortable in the new house.

The two got along fine, no big issues, but they were not and are not close friends. There is no way I'd force two teenagers to live together if they didn't want to.

Parents moving too quickly into new relationships without considering the affect it'll have on their kids (or genuinely healing themselves) is responsible for a boatload of drama we see on this board. If your daughter is feeling the way she is, it WILL cause tension in your home, and your new bf will most likely grow resentful if it’s not managed properly. Your responses to posters who are gently trying to tell you this are telling me that you're more concerned with yourself than you are about thinking of future impact. I don't believe kids should always come first, but considering the amount of change your daughter has gone through & that you're moving in with someone new despite how she feels about it, you're setting yourself up for problems. 

Your daughter is 17, she'll be away to school & starting her own life really soon. Is it really that big of an ask to put her before your need to move in with your bf for a few extra months?  Are you in therapy yourself to reconcile your previous relationship? Have you asked yourself why it’s so important to move in with another man right now? 'Because I want to' isn't a great reason. Is this really in everyone's best interest or would you be better served to ensuring that you're the healthiest version of yourself & are nurturing the relationship with your daughter while building a healthy foundation w your bf before taking a step that will drastically change the lives of everyone involved. Your daughter is important here too & you seem to be glossing over that fact.