sex with my bf

ok so I'm 15 & I've been dating this amazing guy for 10 months, he's literally so sweet. We've done everything besides sex & he doesn't push me about having sex, he really wants it to be my decision. I realized I was ready for sex, or so I thought.. A few weeks ago, we had tried. (Also, my boyfriend is kinda thick) So as we try, we just can't seem to get his dick in me. It happened once that we only got the tip in. I really don't get it. We tried different positions and nothing worked so we gave up. We tried a second time & it didn't work again. I don't understand. Even though we've given up having sex for now, I'd like to know what would make it easier for the next time around.