The only way I could relax was when I was with my children.
I know it's a rare privilege, but if one can really tackle something in adult life that means that much to you, then it's more rewarding than anything I can imagine.
Just because we can't find a solution it doesn't mean that there isn't one.
But the best problem I ever found, I found in my local public library.
There are proofs that date back to the Greeks that are still valid today.
The greatest problem for mathematicians now is probably the Riemann Hypothesis.