I'm middle-aged now and done a ridiculous amount of needless suffering.
I wish I could tell my teenage self to stop worrying as everybody is too busy thinking about themselves to notice or care what you are doing or saying.
I'd say the answer lies within, focus on the breath and the present moment, treat the chatter in your mind as clouds that come and go, the past is done and the future will take care of itself.
I'd say don't cling to anything or anyone. Everything is temporary. Be ok with that.
I'd tell myself not to worry about death because death is part of life. Once you realise you are the character in a play and also the audience and the theatre, things will start to make sense.
One day you will realise you are connected to every other person. The people who are rude or nasty or plain evil are deeply unconscious. When you breathe, wait for the chatter to stop and appreciate every sound and smell and sight. Then you are conscious.
When you see a bird in the sky watch it fly, free from stress. When you pass a tree, touch the leaves. This is nature. You are part of nature.
Processed food will make you sick. Food manufacturers design products to manipulate your taste buds. Buy fruit and veg. Sugary items will slowly do a lot of damage.
Helping others is medicine for the mind. Do it when you can.
Your dad is sociopath. Just deal with it. Your mum and your nan are amazing family people. Just put up with whatever they say.
Be nice to your sister ffs. Stop being a dick.
Alcohol and drugs. It's ok that you feel the need to take this stuff to feel confident but ultimately it's going to lead to ruin. Be comfortable with being shy. People will appreciate you. You don't need to be wasted to be interesting.