Here’s an advice my therapist gave me to build a positive self-image, and I think it works pretty well:
Carry dried peas (or sth. of a similar size) in one of your pockets. Whenever you have done something well, take one of the peas and put it in a different pocket. Look at the peas from this pocket at the end of the day and review what you did.
To do something well does not mean you have to do something extraordinary: It can be as little as getting up in time, if you have problems with that. Or making a healthy breakfast for yourself. Or finally sending that stupid mail you should have send a week ago. Or conquering a panic attack. Or calling a friend. Or being nice to someone/ having a nice little chat after which you feel good…

This technique teaches you that you are able to shape your world and your feelings in small ways, it makes you feel more powerful and it trains the positive voice instead of your inner critic who is trying to put you down all the time.

Try it for a few days and let me know how it works out!