One of the most important things that I do to keep myself mentally sharp is to ALWAYS play with something on the line, even if it's just a small amount.
Look around your pool shooting community and I'll guarantee you that 95% of the "A" players (myself included

) there don't play others--even their friends-- just "for the hell of it".
Extra incentive and material reward helps keep us at the high mental level we need in order to compete at our best on a consistent basis. Thousands of people worldwide can shoot "the lights out of" pool balls, but only a small percentage of these players have developed the mental "killer instinct" required to do it consistently. The physically talented, or "B" players, are the ones that can run a couple of racks in their basement and then lose in the first round of a $5 entry fee local bar 8-ball tourney.
You must ALWAYS play for something, even in "practice", if you ever want to move on to the true "A" level of consistency.
Becoming an "A" level status player isn't cheap-- you're going to literally have to "pay your dues" and play "A" players until your mental game rises to the next level. Then, with time and patience, you should be able to get your money back-- maybe even get well!

Good luck!
Rick P.