I don't think I have ever seen someone change their long-term average happiness more than a small amount with maybe two exceptions:

1. Someone trapped in a truly off-the-charts stressful environment and then removing themselves from it

2. Psychiatric drugs

I've also seen this happen for people who:

3. Leave long term relationships

4. Change careers or go back to university

5. See their parents pass away

6. Have children

7. Lose children

8. Completely change their physical health (diet/exercise/sleep) for the better/worse

9. Loss/change/gain of social groups, or specific friends

10. Gain/loss of religion

Leaving my long-term relationship was the best decision I ever made. It has been 3 years and it still makes me smile every day just realizing how much happier I am now.