I think the mind is like a forest floor: the more you walk paths the deeper they get and the easier it is to walk them again. When I wasn’t doing so great, there was a path falling into unhappiness, and the more I walked on it, the deeper that path got. It’s important to learn how to tell yourself, “No, I’m not thinking about that right now. As a matter of fact I’m never thinking about that; we’re done here.” Eventually, if you let it, that darker path will get covered up with leaves; the leaves will disintegrate over the winter and by spring there will be new dirt covering it. The path is still there but it’s shallow and small and you don’t have to fall into walking it. In the meantime if you build more positive paths they will become easier and easier to find.
One of the biggest realizations I had is that happiness isn’t something that happens to you. It’s a choice.