Ok, I've read about this on here already but I just want to know why I'm only lasting about 3-5 mins. I can last a fair while when jerking it and can usually control my orgasm but when I'm having sex I don't last that long. Could it be due to anxiety?

Also my gf and I probably have about 5-10 mins of build up before we do anything, could that affect it?

There is a buildup, and a release. It is when you ignore this buildup, and the very rhythym you need to alternate to avoid the release, that you "cum too quickly". You know damn well, deep, deep, deep inside, how to stop: GO SLOWER.

Stop if you have to. Slow down. Listen to your body. You know, I used to jerk off to "get off" and "shoot my load" because it felt good. Then I realized all I was doing was wasting energy. You need to learn when youre jerking off and having sex to enjoy being in the moment without rushing to the end. If youre too worried about what she'll think, youre gonna keep fucking up. Listen to the body, it knows.