Hey Vincentp. I dated a guy a few years ago, the same happened for him. He just couldn't ejaculate when we were together, on his own, he was fine. It wasn't a huge problem for me but eventually we discussed it. He said that he wasn't used to being with someone as often and that he felt a bit vulnerable. I guess it is a vulnerable moment. he wanted everything to be right and fun, but concentrated on that aspect so much that he got himself into a cycle of worrying before, during and after sex. He also thought that I was expecting sex every time we were together, which he felt pressure about, It wasn't true, but that's how we felt, so we didn't do it for a couple of weeks. I think maybe you are putting too much pressure on yourself. The guy I was dating eventually was able to ejacuate after a few weeks following our conversation. At 18, you have a lot of experiences ahead of you, enjoy being 18 and enjoy being with your boyfriend and relax!
Just relax man. I've only ever came with a few girls and my present partner and I were having sex for months before i could manage it with her. Now I've got the opposite problem and would like to be holding on a bit longer!
Bottom line is that it'll happen when you're comfortable with someone. You've just got to assure your boyfriend that it's nothing to do with him and ask him to be patient.