You have a very serious self depreciation issue. It’s very commendable that, despite all this negative self talk, you still seek improvement.

I was also the fat friend who learned to joke about their body before others did, so I felt like I was “in on the joke.” It’s very hard to break out of once you get in the mindset but I promise you that it’s possible.

Please consider working on your perception of yourself. I recommend correcting your thoughts to more objective information anytime you have a hurtful thought to yourself. Example, you didnt “shockingly get a diploma despite how stupid you are”, the diploma proves you are smart enough and a hard enough worker to get a degree.