It is hard it’s a vicious cycle with drugs, but the only way to stop it is to get professional help some can quit on their own but not many can. Of course this has to be when your ready to do it. I think everything is a little overwhelming for you at the moment and drugs don’t help with this either as you will struggle then need to use. It sounds like you have realised what you have missed with your ex and have a lot of regret. You rushed into this new relationship a rebound type of thing and that’s totally normal but it’s all got out of hand. I would sit down tell her how you feel maybe take some time alone and concentrate on yourself for a while.
