TMI warning- advice??

Hey so this is my first time posting here- I’m not exactly sure how far I can go with describing things- TMI and such but I’m just gonna go for it!

I am 18 and considered myself a virgin up until last night. I’m currently dating my boyfriend (19) who is also a virgin. We’ve done oral, mutual masturbation, dry humping, and recently tried fingering. So last night, we were watching movies in my dorm and things were getting hot and heavy, long story short we decided to try sex- we did a literal ton of foreplay beforehand (including fingering me for prep) and we had plenty of lube plus a condom. however when he entered me, he kept falling out(?) like I would feel him go in, he’d thrust a few times then I would feel his penis slide out of me. We kept putting it back in but it kinda just wasn’t working. We tried missionary then cowgirl- which worked the most considering he didn't slip out as much. Eventually, we just gave up and I performed oral on him and moved on.

Just as some extra information because I’m not sure if these factors have anything to do with it- I do see a gyno and she said that even during our appointments I was tight- for lack of a better word... I have severe PCOS and am on medication for it as well as the pill birth control- I’ve been taking it since mid-January. We are both plus-sized and overweight too. When he inserted into me, we could both feel it but when he started thrusting he would quickly slip out. It wasn’t painful for me per se- more of just uncomfortable? In the sense that I fitted around him like a snug glove. I’m not sure if it’s a bad analogy but that’s the best way I can explain it without getting too explicit ya’know? I feel the same way when he fingers me, but once k get used to the sensation, im comfortable and good.. I personally don’t think it’s his size- as he’s in the average-size vicinity and has girth, but then again I’m not sure.

I’m not sure if I did a good enough job explaining it but I just want to know if anyone else experienced this. And what we can do to make it better next time we try to have sex again?