Could you seek counselling to help you deal with your mental health issues? Also what about Gamblers Anonymous?
If you can treat some of the other stuff you are dealing with then you might be able to get yourself off the coke. If you are struggling with those other things then I think it’d be a lot harder to stop using it.
I can understand why you don’t want to tell your partner and why the thought of her finding out makes you do it more. It’s very scary and you have a lot to lose.
My partner has poor (untreated) mental health too and I feel he uses coke at times to cope… e.g. if he is stressed or if he feels down… in the long run it is just a very temporary fix as when it’s worn off he is a deeply unhappy person. It’s really a vicious cycle and not the answer at all.