Hello !

I wish I can help you but I'm a beginner.

When I find myself not relaxed, I stop playing for a few second and shake my left hand a bit to relax it.

Another clue is to spend hours doing chromatic scales to muscle your fingers (especialy the weaker ones)

It's boring but it works for me.

Another trick (before playing) is to warm your hands by spending time crossing them each other (sorry for my bad english, I hope you understand what I men).

I hope it helps a bit.

Regards

Patrick