You and I both know that weed can be very psychologically addicting. If you are lonely, getting a little high or quite stoned can temporarily make you feel better -- but as you know, when the high wears off, reality hits you in the face again. If weed is always available in your house because your folks abuse it, that will make your battle for self-worth a tougher one. There's no way around this issue. You must let your folks know how you have been feeling and tell them about your chronic use of weed. They must assume some responsibility for their part in this and be the parents that you need them to be. I know that talking to them about this scares you and I don't know what their response will be. I can only hope that they will wake up and smarten up so that they can help you get better.
You MUST trust some adult to help you. You cannot do this on your own and it's not because you're a weak or bad kid, it's because right now you are too overwhelmed with sadness and despair. Confide in an adult, school counselor or clergy member whom you trust. Let them take some of the burden. They will be honored that you have chosen them to trust. It's the first big step that you have to take. You need a support system and the knowledge that grownups will stand by you as you show how much courage and determination you have.
You were not meant to fail school, to have no true friends, to dull yourself with weed. You were meant to know happiness and joy. There are many people out there, just waiting for a friend like you - people who don't need you to do drugs with them to be your friend. Let someone into your life who will help guide you and support you as you come back to life, to be the girl you were truly meant to be. Please write me when you have taken this first step. I will hold a good thought for you until then.