there are a bunch of different dynamics at play here....how young you both were when you met and STARTED dating, the on again off again pattern, his long and protracted drug use of AT LEAST 3.5 years, and then the sudden turn about and off to rehab.
now you are both in your very early 20s.....and now he's far far away and starting to sketch out a sober life (well we hope anyways). it's "possible" that for HIM a clean and complete break from EVERYTHING that went before seems to be the best approach for this new sober life.
but regardless of his motives, regardless of the WHYs, it is what it is. the past six years were not WASTED, they existed, they happened and are not to be voided. it's the present and the future that is changing for you.......if one party says to the other that they no longer wish to be in a relationship with them, well then.....it's over. kaput. as painful and stunning as it may be........is still IS.
now your focus must be about YOU. about getting you healthy and whole, and happy. that will take time, but it will happen if you devote yourself!