My partner cums quick.

So, I just turned 18 years old and I recently lost my virginity. The first time me and my partner had sex he didn’t cum quick he lasted for at least two hours. But now every time I go over to his house and we have sex he cums quickly? I don’t know if it’s because I’m really tight and it feels good to him or? I just need to know. Also, i need tips on how to ride dick because that’s something I’m really bad at. I mean it is my first time actually doing sexual things but still I need tips😭.