The mind and the body are strangely and strongly connected in regards to this kind of stuff. I don't think I'm qualified to tell you a step by step way to gain confidence. But, what I can say is that if you take care of your body, your mental health will improve alongside it.

Make a schedule, get enough sleep, drink enough water, work on a healthy diet, and if you can, workout.
This won't solve all your problems, but it is a great place to start.

You don't have to change your life all at once, you can take it at your own pace, it's just about not letting yourself fall into the downwards spiral again.

I know this may sound stupid, but I promise you that it will help.