Probably a combination of things, not just one single thing.
1. The mental illness
2. The bleak outlook on life that comes part and parcel with being a liberal and most likely an atheist.
3. Fading career
4. Money problems
Add those up, and it can be a lethal combination.
Still he chose to end his life and bring great pain to those who loved him....regardless of what problems he faced...he’d been in those positions more than once throughout his lifetime. He knew by those experiences what to expect....so he simply quit the race.