Without condoms, I only last five minutes: what's wrong with me?

My girlfriend and I have been dating for over a year. I have never loved or felt this way about a girl. We have been sexually active ever since we started hooking-up and dated. She has the ring, and I like not having to use a condom. She is the first girl I have never worn a condom with though. At first I could last 15 to 20 min of intense intercourse and now I last 5 min tops....