Believe me when I say that I know what you are going through. I haven't used any drugs myself, but due to my anxieties and mental disabilities, I often feel alone in the world and feel that time is constantly against me (for the people that know me personally, know I am quite obsessed with time). Life isn't exactly pointless, it is just that life becomes pointless to each individual after death; though your actions may still have an effect on the people still alive, the time you spent on the earth while living won't matter to you.
If you really don't care for yourself that much, and can't bring yourself to care, care more about the people you care about instead. Rather than doing things for yourself, do things that benefit other people more. There will always be somebody that cares for you in at least some way, especially if you have real close personal friends. Even if it is a stranger, just saying 'hi' or hearing somebody out that has had a tough day can make it better for them.