This is a major question. I'll provide just a few of many brief ways:

1. Improving insight into the illness (recognizing symptoms easier/quicker)

2. Challenging dysfunctional thinking (this includes many things).

3. Action planning on how to cut episodes short.

4. Recognizing triggers, preventing/avoiding them, and/or preparing/managing them.

5. Learning to forgive self and others.

6. Making life choices that help ensure wellness (includes a lot).

7. Acceptance and stigma fighting.

8. Accumulating a whole toolbox of coping techniques and working to master them.

9. Processing... processing... processing. There's a lot to this!

10. I can't stop at nine. A therapist is a great emotional support and general advice giver. The more support, the higher chance of wellness.