The key to happiness - or that even more desired thing, calmness - lies not in always thinking happy thoughts. No. That is impossible. No mind on earth with any kind of intelligence could spend a lifetime enjoying only happy thoughts. They key is in accepting your thoughts, all of them, even the bad ones. Accept thoughts, but don't become them.
Understand, for instance, that having a sad thought, even having a continual succession of sad thoughts, is not the same as being a sad person.

Matt Haig