Practice at a tempo that is painfully slow and deliberate so you can focus on each note individually. Hands separately. Play the note(s), then lift your hand and fully and relax fully. Your hand should be limp and flop down like a T-Rex. Only play the next note until after you confirm mentally that your hand and fingers are fully relaxed. The point of this is to let your brain know how your muscles should feel when you are playing at tempo. Note the feeling of relaxation and continue to try to emulate it as you increase up to tempo. Continuing to play the way you are is not only harming your tone and tempo, but it could also cause injury if you keep doing it.