Just going to put some examples here, in case anyone needs them:
Compliment: You write such great angst! I was in tears after that last chapter.
Compliment + self-insult: I could never write angst like you. Mine never has that gut-punch that yours does.
Compliment: You look gorgeous! That's such a pretty colour on you.
Compliment + self-insult: I wish I had your fashion sense. I just wear whatever smells clean.
Compliment: This piece is awe-inspiringly good! How long did it take you??
Compliment + self-insult: Your art is so much better than mine. I could practice for a million years and never get close to this.
The thing about insulting yourself while complimenting someone else is that you might think that you're making your compliment sound that much better because you're increasing the distance between their level and yours. But what you're actually doing is refocusing your compliment away from them and onto you and your insecurities.
Maybe they'll notice and maybe they won't, but you will. Eventually, all of the insults you've given yourself will make you feel bad about that other person's skills and now, instead of a mentor or a colleague you have someone you're jealous of. Or at least, that's how it tends to go for me.
Keep the compliment focused on that person and their skills or presentation or process etc. And if you want to learn from them, ask them for advice. It can be hard to notice when you're falling into this kind of habit - I thought I'd gotten past it but here I am again - so just do the best you can ❤️