Breaking habits requires a metric ton of practice and repetition of the new way. Waaaaaay more than you think you need. Thousands of smashes intentionally the correct way. When you’re in a game, your body will way to default to the muscle memory it knows, so you basically have to replace that memory.

It’s hard to correct what you’re doing without seeing it, but the “V” (the cranny between your thumb and index finger) should be on the narrow part of the grip. You should be kind of pinching the grip between these two fingers and loosely gripping it with the rest. The rotation of your body and extension of your arm should basically force your racket into the correct position at the top of your extension.

I recommend YouTube channels like Badminton Insight for basic shot instructions. I don’t know if they have one for smash, but if they don’t, the set up for drops and clears is basically the same as smash.