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You may not believe me, (I probably wouldn't have) but this is the cure;
Listen to your thoughts, but understand that you don't have to believe your thoughts, keep challenging yourself on every negative thought, try not to let any get through, they stop trying after a while and you will have them on the run.
Live in the moment, stop thinking about the past or the future, you don't know what will happen you can't change what has happened.
Face discomfort, don't avoid it, don't escape it, survive it, find the satisfaction that comes with facing up to your life and baring it.
Learn to have faith, believe in yourself.
I was a major depressive and an addict of all kinds for decades, I thought I would never escape it.
I encourage you to read my follow up pieces, The Voices in My Head & Playing With My Emotions.
Good luck. You can msg me if you want to talk.