Before the Difficult Days
It is easy for young people to ignore how short life really is. At 21, 80 looks like forever. "So teach us to number our days, that we may gain a heart of wisdom" (Psalm 90:12). Even if you do live a long life, though the longer you use drugs the more that is doubtful, it doesn't mean the last days of your life will be active ones.
"Rejoice, O young man, in your youth, and let your heart cheer you in the days of your youth; walk in the ways of your heart, and in the sight of your eyes; but know that for all these God will bring you into judgment. Therefore remove sorrow from your heart, and put away evil from your flesh, for childhood and youth are vanity. Remember now your Creator in the days of your youth, before the difficult days come, and the years draw near when you say, "I have no pleasure in them"" (Ecclesiastes 11:9-12:1).
God intends youth to be a time of fun, not worldly "fun" but true fun that brings true joy. That is why Solomon said to "put away evil from your flesh;" nothing saps the fun out of life like sin. But if you put it off and continue to insist on chasing after drugs and other sins, it will eventually catch up with you. Even if you somehow manage to leave drugs behind later, it will be too late to enjoy all that you could have had in life.
I want you to have a great life, one that you can look back at and be satisfied that you did well and lived it to the fullest. Yes, you've sinned, but is that a reason to stay in chains or a reason to break free? Break out of the cage Satan has you in now, leave the sins behind, and start really enjoying life again!