Little To No Sensation/can't Orgasm

I'm not sure what my problem is, so maybe someone can help me out.
I've had very few consensual sex partners, all of which I have barely felt any vaginal sensaton with.
The first time around is always great. I can feel everything in every position, but then each time after its as if my vagina turns off. My current boyfriend and I had sex for the first time a few days ago and it was great. I had no issues, but when we did it again last night, I could barely feel anything. It was nothing like the first time and I only had a bit of sensaton in positions that directly hit my g-spot. With that said, my boyfriend has a fairly small penis. He's no more than 4-5 inches when erect, but he seems to know what he's doing judging by our first time being intimate. He's the first person I've been sexually comfortable with after experiencing two sexual assaults over the course of four months, so I'm sure that's a part of why I struggle to feel anything. I've also never experienced a vaginal orgasm with anyone before and my boyfriend claims that he's been able to make all his previous partners achieve orgasm. He and I are so desperate to have a normal sex life after we've been through our fair share of abusive relationships, but I keep feeling like we'll never reach our potential if I can't figure out what's wrong with me.