Maybe taking some time to confront some of the past traumas in therapy will help. Be brave, it will be hard but hopefully if you can confront some of the things that make you have negative thoughts in a safe environment like therapy, you will be able to overcome them and have more positive thoughts. Until then, keep up the blocking the thoughts when you are not in a position to deal with them. Try some slow breathing exercises to keep your thoughts under control.
Congrats on the weight loss. I still have to lose about 10 kg but its not happening.