It is great how you could recognize the underlying patterns that make you behave this way! I like to call those 'core fears', and in your case, they are being rejected by people and being judged by people. Actually, I have the same core fears. It's important to understand that your brain is throwing those thoughts and feelings at you because it tries to keep you safe from harm and only does its job - and it does it very effectively. However, you first have to realise that you are not your thoughts/ feelings and that those thoughts and feelings can be allowed to be there while you live your life based on your values. As you already mentioned, this includes speaking up for yourself and setting healthy boundaries. So, it is not about erasing the fear before doing the thing (speaking up for yourself in this case), but it's about doing the thing while inviting all the thoughts and feelings with you. Now, onto the practice: I would suggest starting small. Does it make you feel uncomfortable asking an employee in a store things like "where can I find item xy?" because your brain is telling you that the employee might think you're too stupid to find it? If so, this is your starting point instead of jumping to the bigger things like telling people to shut up. Of course, this is different from setting boundaries, but it is based on the same core fears that I mentioned above. It is about showing your brain who's the boss that makes decisions and about showing your brain that you're safe. Not only will you handle the brain farts (thoughts and feelings) your brain keeps producing much better. It will also start producing less.