Why can't I for my girlfriend?

I'm able to masturbate to without a problem, but whenever we start getting rowdy, I can't seem to for her. The first time we made out (which included some , but no under the clothes stuff or anything big) I actually came, but the next time I couldn't even get hard for her when she was giving me a , and since then I've been able to get it up for her pretty easily, but I can't seem to finish. She's tried s and s and although I can feel myself getting close (as in, I could finish myself off at that point) she can't get me over the edge. It's really annoying because she thinks it's her fault even though I tell her it's not. I know this is probably all mental and just kind of a block in my brain that prevents me from just letting go, but it's really putting a strain on the physical part of our relationship. We're not having yet, so we're limited as to what we can do. The annoying part is she won't let me finish myself off if she gets me all the way there, because she wants to make it happen and gets offended if I try. She also doesn't like it when it looks like I'm making myself try too hard to finish. I've heard that you shouldn't go into it with any expectations and be fine with not finishing, but I'm kind of embarred because I've never heard of this happening to guys and my friends are able to come just fine with their girls. Can someone please help? I just want to give her what she wants.