First, the good in quitting has to outweigh your desire to use it. If you have no real reason to quite, you never will. You have to have a reason so you can think about it every time you want it, something bad that comes from it, or something good that will come from not using it.
It’s mental. Same reason why there are fat people and people who say they want to quite smoking and never do. It’s all about self-control. You ultimately have control over your mind and body, no matter what the addiction or urges.
You try to quite, then you start thinking you need it, you will try to convince yourself somehow that it’s ok, just one more time, you’ll quite tomorrow etc. once you realize there is no one more time no matter how bad you want it, or what reason you tell yourself, the answer is no, you’re done with it.
It all comes down to the pros and cons of quitting. Smokers tell themselves when they want a smoke, “why quit? I’m going to die anyways. It’s only a few bucks a day, what is it really hurting? etc” it’s a mental process, if you’re convinced someone is going to go to your parents house and rape and kill your mom if you use drugs again, you will quite forever correct?