Just practice your swing without actually hitting anything. Pay attention to your form and where your racket head is facing at the point when you should be hitting the shuttle. Make adjustments based off that and keep practicing till it becomes just muscle memory.
I had the exact same problem. You just have to catch yourself constantly and try to hange your grip. Took me about 4-5 months to change conpletely. Bad habits are hard to change :/, there's no quick fix except just have yourself or your friends remind you or point out when you are in pan handle
Be reassured that learning the right grip is not easy! It takes time. Don't be disappointed if you appear to make slow progress.
The grip itself is trivial to learn, of course; the hard part is learning how to use it. The correct hitting action is very different from what most people naturally do (panhandling).
You may find it helps to concentrate on some of the simpler shots, such as a lift from the net. These are less technically complicated than (say) a clear or smash, so it's easier to focus on the grip and the arm movement.
How about just bouncing the shuttle up in the air, on a racket? Stick to the forehand side for now. This is a simple exercise, but surprisingly useful. Keep your grip relaxed, and allow your wrist to bend back a bit before gently hitting the shuttle upwards. Try for control first, so you don't have to run all over the place to catch the shuttle! Then try tightening your grip as you hit the shuttle; experiment with this to gain control over how high you hit.
With regard to the clear or smash, I have an article that might help you learn how to use arm rotation with the basic (forehand) grip.