Oh dear. I couldn't read and run. I have some similar background so I hope I can help. Basically all the same minus the pregnancy.
Firstly, I'm sorry for whatever you've been through that got you here, and that it's now affecting you again in this way. It is rubbish and such an isolating experience.
As blunt as this sounds, you have to deal with this openly with your OH. You can't just hope it will go away, or fight your way through it. I actually asked my boyfriend for a 6 month break (in the end 3 months was enough); I needed to get out of the cycle of having sex because I felt I had to. I had to find my no. I also think it helped him shut down his own sexuality and not feel so disappointed or wondering "will it happen tonight". He was so supportive, I would really hope your OH will be the same.
I think it's important you learn to tell your own story. The fact your OH knows 20%... Of course you only should share what feels right, and when it feels right, but I think a big part of owning what happened to you is telling the person you spend your life with. Your sex life going forward may look very different, and you and he need to have a clear and transparent understanding of your experiences, and why your sex life is the way it is. But I think that's a goal to work towards, and can be done little by little over time as you feel comfortable.
There are some charities who offer free or sliding-scale therapy. Google your situation (for example, Rape Crisis might do something like this, or charities that work with survivors of childhood sexual abuse - I think AVOCA is one of them...)
There are also some wonderful self-help books out there. At a more general level, The Compassionate Mind approach to healing trauma by Deborah Lee has some really practical skills, as well as decent coverage of trauma as a concept. I would definitely recommend "the body keeps the score" to understand how trauma impacts the body (and if your OH reads, you might ask him to read it too so he understands more of your experience). Its a long slog though. On a personal note, if the trauma you're working through is from childhood, I would highly recommend Ellen Bass' The courage to heal.
There are also plenty of books around reclaiming your sexuality for those who have suffered trauma, but I imagine it's far too early yet for you; you'll know when you're ready to look into some of these.
Since I've been working through this stuff, I've shared a lot more of my past experiences with my partner, once I felt safe, and we've had a lot of conversations about our current sex life, things that trigger me, what he can do to help, and what makes things worse. These are all much-needed conversations, and you WILL get to a better place, but it is not a quick journey. Nor should it be. You have your whole lives ahead of you to enjoy great, fulfilling sex together; now is the time for you to focus on healing your wounds and finding yourself again. Clear and open communication with your OH, and lots of patience and kindness with each other, will be the tools that get you there.
I really do hope some of this helps you; I'd recommend starting by writing a little note to your OH (if that feels easier than an actual conversation), sharing some of what you've written in your OP, and some of what memories you're struggling with if that feels safe to share, and asking to take 6 months out from sex so you can focus on your healing journey. Order some books for self-therapy, and slowly take it from there.
I would offer for you to PM me but I am terrible at checking my PMs! 
Take care of yourself and your little growing baby 
