You can treat yourself to both sugary treats and happy little thoughts as a reward for behavior you want to reinforce.

You can choose to entertain those happy little thoughts, even if it takes effort, even if you later revert to depressing thoughts.

Thoughts repeated turn habitual, even if they didn’t feel sincere at first. (For both good and ill.)

Those happy thoughts don’t have to be delusional. Slightly more cheerful thoughts can suffice. Frequency matters more to habituation than anything else.