Newbie Advice

I actually am quite new to being with another guy except for my best friend growing up.
Ww both date girls but have been having oral sex together since middle school.
Recently I hooked up with a guy who's quite older, 37 and have gone places sexually that I never thought I would. I actually let him fuck me and his cock was probably too large for me and my first-time but it's totally changed me.
I've been with him like ten times in the last two weeks and I'm afraid that he's going to ruin my ass. What I'm asking is if I keep letting his large cock keep fucking me like we've been doing is it going to damage me in any way? He's 8" and kind of thick. I seemed to tighten back up each time he's fucked me by the next day but it seems like it's getting easier and easier to penetrate me. How do I keep from getting permanently stretched out???