Do you ever feel like you are a piece of shit for not letting your s.o. find someone better?

Not every man's. But I walked away from mine. He is free to find whomever he wished now.

Don't be so hard on yourself. The reason why he's with you is cos he wants to be.

No, some see the beauty and complexities of the lady they are with, even if she doesn't. They are wanting to love her into Life, into the person she could be with her potential not locked away by self-doubt.

No, not every guy. Just most.

He wants to be with you. Try and accept that. It seems that you’re more unhappy than he is about being together.

From experience, saying this to him and yourself will push him away.

No. When I'm in love I have absolutely zero interest in pursuing other women.

i was in your man’s situation once but I had no interest in such a blessing; even for men ik some whose gfs allowed threesomes etc but they wanted monogamy

No. Men are simple creatures of habit. Give them a little attention, make them feel needed, give them a little sex and they are happy campers.

I personally, could find somebody "better": someone who has a nice family(hers is crap), someone with a real job(also crap), someone better with money, someone more ambitious, more like a gorgeous brunette supermodel, etc.

I'm not. She has plenty of faults, but also plenty of strengths: she lets me live my life, rarely says anything abut the time and effort I spend on my musical pursuits, is a fantastic cook and a great snuggler. She knows or has met all of my friends, male and female and makes no demands on my time(I prioritize her). It's the highlight of my day to spend time with her.

Would I give that up just to smash some other girl?

See my point?

Perhaps he sees it as a line you drew in the sand, or a challenge, like "go f someone. I dare you," anticipating a self-fulfilling prophecy if he does. Are you looking around, too? Wouldn't that make sense if he was? What's your end game with this?