hi everyone.. I need your thoughts, please.. please forgive me it's gonna be a long post

I am writing this at a peculiar time in my life; I have been on this board for a number of things that stress me out and make me unhappy, and I didn't want to add sex to the bunch but.. right now everything is going better, except for that. So... background.. I am female, 26, my upbringing was very unnatural 'cause my mom always depicted sex as dirty and I took her word for it, I absorbed her point of view, even if books, movies, people, nature, etc suggested otherwise, that is was a natural thing and others liked it.. so I often felt split in two. I didn't want to wait until marriage 'cause I felt that sex is a major part of any relationship and you should seek compatibility in it (the wild half of me being all open minded), but I didn't want to "give it away" either (the quiet half being all serious and virtuous), so I only had sex with one guy, my first serious relationship. I wanted to be able to trust him and let go. Yes I trusted him and he was the perfect guy to be my first, good guy, in love with me, gentle, everything good, but I am not sure I really "let go"... I have to say I didn't have fun much, the first time we really did it, I thought.. is this it? Ew. I loved this guy, he was the best person in the world, but maybe I thought it was the lack of passion causing that. We broke up a long time later for various reasons.. then recently I met a girl and fell in love with her (it was not a shock or a change in my life, I have always thought I was able to fall in love with whoever, gender is irrelevant to me).. the first time I kissed her it felt so natural and awesome, so no pressure about her being a girl, and I liked her from the very start (while she didn't notice me at first), so this time "passion" was not a problem, I am very much attracted to her phisically, I think she's beautiful and I "wanted" her.. we got together and the first time we had sex I was all nervous, I was afraid the not feeling anything would come again. And it has. I mean I love being in bed with her, the contact, and everything, I just can't get in the right mood somehow.. at least, not at the right time when we're in bed together, I feel so cold and detached..
I want to be different.. She has no problems at all and I feel like I am ruining a part of us.. I just.. am not like that.. I love kisses, caresses.. that's all I think about.. and then it stops. I want to kiss her and I want to caress her, but I have to -decide- to do more directly sexual stuff....
I want to be healthy, 'cause everything is perfect this time, everything but this.. I don't want to be the girl with a problem.. I don't want to be cold. I want to feel good and make her feel good and just be happy with her.
I don't have much sexual experience so maybe you can see something I can't.. maybe you can help me understand.. god, I feel so guilty :'(