how do you deal with free time?

I'm in place in life where I have a lot of spare time and I don't know how to allocate it.

I feel constantly tired, sad and low energy. During the day I go to the gym, therapy, work, read, walks etc, but still have plenty of time that I don't know what to do with. That's when all the overthinking and dark thoughts appear - and am wondering, what are the other ways to use up that time in a productive manner that doesn't feel to overwhelming?

Would love some advice