I don't like having sex.

Hi, I'm 18 and I've been with my current partner for 4 months, and I don't enjoy sex at all. It gets to the point where I get bored and just want it to end. It's not that I have erection problems or anything like that; in fact, I get good erections. But it gets to the point where we're having sex for an hour and a half, and I don't finish. She's left dry, and I'm still not climaxing. I did some research, and apparently it's masturbation. I started cutting back a month ago and stopped completely two weeks ago, but nothing. I still don't feel anything. I'm starting to think it's psychological. Does anyone have any advice? I feel frustrated because other people enjoy it so much, and for me, it's so boring.