At the end of each day, spend 5 minutes in quiet reflection.
When ready, record in a notebook 3 true and positive things that you did. The 'doing' is the important bit.
Keep it going for a few weeks and let your 'true and positives' build up like a brick wall.
A brick wall of confidence. It starts with little things. Define a goal. Complete a few tasks. Brighten someone's day. Put in a solid practice session. Get that gym session done. Say 'no' to a second chocolate brownie, run that first mile, go that extra step. Making that phone call. Have that difficult discussion. Pay that bill. Try something you've never tried or one thing that's going to make you a bit uncomfortable.
Go on, you keep going now....