Hey, I had this problem of negative thinking, I was a complete pessimist, but Ive come to learn that the negative things in my life that have happened have only turned me into the man Ive become, and it has made me experience many things that have changed me
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Another thing that I have done is everytime there was a problem, I would look at the bright side, even though with some things you may think that there may not be one, however you end up changing due to every negative thing that happens since it makes you react to it in a better way the next time. I have no idea if this makes any sense at all lol but good luck!
Surrounding yourself with the right people also affects your thinking, you mentioned you are an introvert, this may withdraw you from communicating with people about your problems, if your communicating with people who give you negative feedback, then try to get advice from somewhere else
, I hope these comments are helping you.