Sorry you are going through a confidence crisis .
It's easy for a t to get annoyed about it and be judgemental because they have been there and worked through it already.
It's easier to think negatively than it is to think positively about ourselves and especially if we have been told negative things to us of course we will believe them.
It takes time to rewire your brain into thinking only good about yourself growly cat and to filter out the bad stuff bit Rome wasn't built in a day and with baby steps it can be done.
How about telling yourself something true about yourself for a day, like you are smart or beautiful or whatever it is you know to be true and when that negative thought creeps in just throw it out and say that's false because visible know this is true.