There are proofs that date back to the Greeks that are still valid today.
But the best problem I ever found, I found in my local public library.
I really believed that I was on the right track, but that did not mean that I would necessarily reach my goal
Always try the problem that matters most to you.
However impenetrable it seems, if you don't try it, then you can never do it.
The only way I could relax was when I was with my children.