Hard time having sex

Today my on-again/off-again ex and I wanted to have some sex. It’s been a while since I’ve actually had sex but I didn’t think it would be this hard. We were on the couch making out and doing oral. Then I climbed on top of him and we literally tried putting his penis in me for like 10 minutes and it wouldn’t go at all. He’s pretty big, so I’m guessing my hole was too tight for him. What can I do to make it fit next time or at least some tips or different positions to try?!