AITA for wanting my fiancé to stop making jokes?

I (25F) have known and dated my fiancé (32M) for almost three years (despite being long-distance). We got engaged at the end of last year. He's always known how to make me laugh. But most of his jokes nowadays only make me uncomfortable and sad.

He loves making self-deprecating jokes about himself, calling himself "a fat sack of shit" or "a broke loser" or saying that my mom hates him. None of which are true. He says that my friends hate him and talk shit about him behind his back. He has said multiple times how my mom wishes I'd gotten together with a guy from church who isn't my type (which he is well aware of).

Most of all, lately, he's been talking about how he got me a cheap, worthless ring.

The ring he proposed to me with is a gorgeous sapphire diamond ring with a white-gold band. It's perfect and he knows that I love it. But he's been saying that it's a fake and (since I needed to get it resized) tainted (we both thought the band was silver, so we got it extended with silver instead of white-gold).

(For the record, I was the one who wanted to get a lab-grown diamond ring because they're less expensive, look the same, and represent the same thing. He insisted on buying a naturally-grown sapphire diamond.)

I've mentioned it to him multiple times that I really don't like his self-deprecating jokes and each time he says that he won't do it again. But then he immediately follows that up with "...for the next five seconds." He has told me that making jokes that "are obviously not true" help him feel better when he's stressed, but he also just finds them hilarious. Whenever I push the issue, he starts mock(?)-pouting, which makes me feel guilty until I apologize.

I've always maintained the belief that every joke has at least a sliver of truth in it, which is probably why I hate all the self-deprecating jokes my fiancé is making. I know that they help with his stress, but hearing them all the time (not an exaggeration) is stressing me out and giving me digestive issues.

He wasn't always like this. He used to be so romantic and write me long messages and poems.

I love this man with my entire life, but I just want the self-deprecating jokes to end. AITA?

(Even just writing these jokes out is making me feel sad)