Stop competitive online play..... that wont do you any good if you dont like stress.
Stay with rewarding single player experiances, that you enjoy to play.
"The Witcher 3 has become less fun and is beginning to feel like a punishment, a chore I have to endure before I can move onto anything else."
Thats when I usually move onto the next thing, and just put it into a "get done at some point" pile. If gameing feels like a chore, its not healthy or fun.
De-stress & cheer-up?
Long warm baths,
takeing your time and enjoying good meals (it can be fun to try new recipes too)
and I rather enjoy reading a good book too, just snuggle up under covers, layback in bed, and relax with a book.