Have you thought about therapy? Sometimes it is hard for us to rewrite our brains from years of habitual self-criticism and it helps to have an objective point of view to help us through.

I've tried affirmations without much luck but a lot of people benefit greatly by using them. Post them on your mirrors, fridge, etc.

Personally, I try to use thought stopping by just saying "stop" and then changing my thought to a more positive one or direct it to something in the present moment. It takes some practice but helps keep me from the negative thought patterns. I use it for obsessive, negative thoughts.

Practicing being grateful helps, too. List three things each day that you are grateful for. Replacing the negative thoughts with grateful thoughts is very helpful.

Good luck.