It sounds to me like you just decided that because she wanted sex that you would do it. Did you ever stop to think about whether YOU want it yet? I mean, of course you want it... but are you really ready? If it was such a surprise, maybe it was a good thing that this happened. You need to make the decision that you are ready when you're not in the heat of the moment. That way, you won't wake up the next morning wondering if you went a little too fast. Just spend some time thinking about whether or not this is something that you're truly ready to experience and share with your girlfriend. If you're secure in your decision, it will make all of the physical stuff much easier, and it will make the whole thing much more meaningful.
"The future is no place to place your better days."