You are a good person, and I wouldn't be writing this if I thought you were some incorrigible cretin without value to society. Still, the path you are on is a dangerous one, and is ultimately a journey fraught with negative consequences to your life, health, and future.
I have known you a long time, and it pains me to see someone with so much potential wasting it chasing cheap chemical highs. You have opportunities and talents of which many other people can dream, yet your time is spent either getting high or thinking about the next high.
You have not always been this way, of course. I am not sure how long you have been experimenting with drugs, but I recall a time not so long ago when you were fascinated with knowledge and you imagined a future where everything seemed possible.
Today, you are unhappy. You are in a place you do not like, but you have not figured out that drugs make this situation worse. The momentary escape you derive from getting high clouds your thinking, and allows you to forget your troubles for a couple of hours.
You are programminmg your body to expect a chemical response: you feel bad, so you get high, then you come down, you feel bad, and you get high. This cycle only gets worse, and it takes larger amounts of chemicals to provide the escape. At some point, of course, your need for drugs becomes more than entertainment or escape, and you become addicted.
Only you know where you are on this addiction continuum, but I suspect you are not even honest with yourself yet. You rationalize this behavior with delusions that getting high is some sort of spiritual quest, or that you are just having fun, or that your life is so stressful that you need a break once in a while.
You don't see where you are headed.
The classic signs are there - declining grades, missing work, constant financial difficulties, irregular sleep patterns, irritability - but you have not yet made the connection between your drug use and these other occurrences. These are just more reasons to get stoned, to make the unpleasant things in your life fade away for a bit.
I have no illusions that this letter will make much of a difference to you, but I would be remiss in my obligations as a human being if I just closed my mouth and said nothing. I have seen too many good people get caught up in the cycle of self-destruction, and the last thing I would want is for someone I know and love to get sucked into this trap.
Good luck to you, and I hope that you learn - sooner, rather than later - that the road you are running down leads you to little but misery and pain.