I think I have premature ejaculation or something?

Okay, so i'm 16 and my girlfriend and i are sexually active. We have had sex quite a lot, which is why i'm a bit worried. I still come within like a minute the first time we have sex in a day. But then we wait like 15-20 minutes and go again, and i can last for like 20 minutes. When we first had sex, and i came that fast i thought it was just because it was the first time, and that after that time, i would get used to it and be able to last longer. But i'm still having this problem and its really annoying. Also, i'm not circumcised and i've heard that if you aren't, you have more nerve endings so you are more sexually heightened or something? Is it just because i'm young? Or can i do something to fix this? Please help