^^
Every time you say i quit its a step closer to actually doing it, even when you relapse, in that relapse is another lesson you'll have to learn.
You've seem to made up your mind, you want to quit, now its time to do it.
But the descision alone is not enough, willpower will get you nowhere if you don't have a gameplan.
Write down why you want to quit, what it will provide for you, why you don't want to use anymore. Write down what you think is the cause to abuse drugs, it can be because you have no goals in life, perhaps you are restless, anxious, depressed, you see the wrong people, etc etc.
Your pharmacologic, psychiatric, social and environmental situation needs to be safe and stable, before you can live sober. If you can see what is wrong in those, take on the challenge to improve them, so your attempt to get sober has a much higher chance of succes.
Take this on step by step, the first one is to provide yourself good sleep and excersise, walks, daylight and structure, healthy food and a clean house, daytime activities.
Thats the foundation.
Don't be affraid to get help from a social worker of psychiatrist, its no personal defeat but only shows you're mature enough to do what is needed to go forward in life.
Good luck