My girlfriend wants to take our relationship to the next level but I don't think I'm there yet?

My girlfriend and I are close, and just recently she told me she wants to be a bit more "intimate" and take things to the next level. I said I'll think about it, but I don't think I'm ready for that yet. I'm 22 but I didn't get to choose who I loose my virginity to if you know what I mean and I havn't been intimate with anyone else since. I was very fortunate to get counselling, so I think i can deal with the actual feeling of doing it. But I don't think I'm emotionally ready to cope with something like that yet. I don't know it's hard to explain. I don't want to lose her over something like this. I know people in my situation just do it, even if they don't like it for their partner, so their partner don't miss out on anything. I don't know what to do. If I don't do it, I fear loosing her. If I do do it I'm afraid my PTSD might play and hurt me and her. Any Ideas? Constructive answers only please. The last thing I want to hear atm is some cyber thug insulting me