How do I stop being so self defeatist?

I would love to get some advice from someone optimistic (though I probably shouldn't be on Reddit asking this lol). I just want to know how I can stop being so self-defeatist? I'm from the U.S. Obviously I don't have much control over the shitshow in the office right now but I want to become an optimist in my own life that I can control.

I’m 21, still living at home in a very religious household, no license can’t afford therapy, never been in a relationship, low self-confidence and self esteem, and much more. I’m tired of feeling like my life is over and I'm tired of constantly thinking the worst is going to happen. I’m constantly anxious and always in a fight or flight mode. I can't remember the last time when my stomach wasn't constantly in knots. How can I fix this? I do not want any religious advice, I already hear enough of it from my father and if it actually worked, I wouldn't be where I am now. I already don’t have any socials except for Reddit and YouTube and I use YouTube as a source of comfort by watching some of my favorite creatives.

I want to get out of this defeatist attitude. I want to thrive and say “fuck you” to my upbringing, this world and my past and circumstances. The question is, how do I get there? How do I have a better outlook on life and stop thinking that the worst is going to happen? I want to wake up not thinking that something is going to go wrong. I want to be able to let go of everything. How do I go about doing this? I would love to hear stories from people that were able to change their own ways and thrive. I’m just tired of feeling like I'm ready to give up, I want to thrive, I want to prosper and I want to live a good life.