Anon,
It's absolutely normal. First, learn how to put on a condom properly. You do not need to change it after ejaculating. Make sure that you leave a small tip of extra rubber at the front, without leaving any air inside. This makes space for the semen to be stored. You need not remove your penis even after ejaculating. At your age (I'm assuming you're young), kisses and caresses could help you erect quickly after ejaculating (a few minutes, am I right?) Your second try will last longer naturally (I won't explain how parasympathetic nervous system works, I don't know myself. Go and read Wikipedia.).

About lasting longer... It depends largely on how you approach the problem. Asking her to stop so you can catch your breath before you ejaculate is a good way. Try to tackle the problem together (I repeat this over and over). Also, learn how to control the muscles near the anus. You can reach orgasm and yet hold ejaculation if you compress the point between your anus and the back of your testicles. Contracting the muscles around that area will hold your ejaculation, and allow you to continue erect even after an orgasm.