Do you ever wonder if the drugs robbed you of your capacity to tolerate life without them? Being exposed to more intense or unnatural sensations can make the brain find life without the drugs to be unacceptable.
Of course you mentioned being depressed since age 13, so I'm not sure if this is the case for you. Perhaps you felt what it was like to be truly free from stressors, and you felt nostalgic because you "remembered" what it was like before them, even if you've never actually experienced this.
Sometimes I feel ideal imagined states of happiness, memories that I remember incorrectly as being better than I know that they were, and future goals promising me contentment as nostalgia. None of it is genuine, it's just my mind tricking me.