What can i do to stop doing this bad habit?

Wrap butt of cue where you grip with something that reminds you not to clench the cue. Your fingers should remain loose and acting as a flexible joint. That is so you don't move shoulder joint and drop your elbow (as required).

Not something as radical as barbed wire, but maybe double sticky tape, at least for a while until your brain gets trained to stop that bad habit.

Another idea is done when you pause after the back stroke. At that point you can tell yourself "Keep grip loose!" when you do the forward stroke.

I'm right handed, and I did the same thing. I found that putting my left hand on the back of the cue, and using my right to aim the shot helped a lot. When I was in college, I could go into a bar, play pool for beers, and drink for free. So I guess I'm saying try switching hands.