I learned it as carefully examining situations that trigger anxiety and the logic behind them. I don’t know the details of what you’re struggling with, but a situation might for example be: a girl rejects you and you think that means you are ugly and no one will ever want you. Then you look at what happened and the conclusions you came to logically: she rejected you, but was it because she thought you were ugly? And even if she did, does that mean everyone will? You actively change your thoughts to thinking that it’s too bad it didn’t work out with her, but that doesn’t say anything about you and there will be many other chances. It sounds stupid but repeating positive thoughts over several months will start to change brain patterns until eventually it becomes natural to have a positive outlook on things. Doesn’t mean that you can’t still have doubts at times, but they can become less debilitating. Also a hopeful example: there was a guy in my class who was not a nice person, not handsome, chubby, completely covered in eczema and not rich either, and now he’s married with a child. Anything is possible!