He doesn't last long, am I doing something wrong

So my boyfriend of 2 years and I recently had sex for the first time. It was my very first time but not his. So lately whenever we do stuff he hasn't been lasting very long and feels SUPER bad about it, I am left extremely unsatisfied but can have the heart to tell him. He thinks the reason he lasted way longer in the past was cuz all the girls he was with he didn't love or have any feelings for, a fling to say. He says he loves me with all his heart and I believe him cuz we have been through a ton together, as friends and dating. I feel like I could do more but he's my first so idk much, am I doing something wrong??? Help?

This is us at prom this year btw