How do you quit smoking ganja if you've been doing it a couple of years already?

Mutual support helps: Consider attending a meeting of marijuana anonymous.

Collaboration: 12-step groups do help with support and practical suggestions for everyday life. But to avoid relapse, you also need to look at the issues that led to your use of this substance. Common ones include various kinds of anxiety, post-traumatic stress, etc. A psychiatrist can help you explore these, and if there are conditions that would benefit, can recommend treatments for them. Good luck to you!