Totally agree with @strawberrysakura:
and the other PP that I am horrifed by the bees being so harsh. Many people do not have access to other sources and people that they can talk to, and so they come here, rightly or wrongly, for input, guidance, comfort or just to vent and share their problems and darkest thoughts. And readers of the thread take that as an invitation to have a go at that person, and that just isn’t right. Even if it is a stranger confiding awful things, that confidence is an honour, and I really do think we should approach these things with a lot more sensitivity.
OP, you need real-life help and someone you can trust and talk to on a regular basis about these thoughts you are having. Substance abuse is not something you can generally tough out on your own; your GP should be a good place to start as far as that is concerned. Thoughts of suicide and hurting yourself need to be taken very seriously; there are toll-free lines you can call to talk to someone. But if you have not already done so, you definitely need to confide in your fiance. He needs to know that you need extra emotional support right now and that you are hanging on by a thread.
It might seem like too big a mountain to climb right now, but putting some distance between yourself and your parents and your in-laws is going to give you that sense of autonomy that you crave. If they are creating drama and you are getting sucked into that drama, it is going to be very difficult for you to feel better. Someone with the mental strength to resist it might not be so affected by all the bickering and drama about the wedding, but you are not at that point, so I would suggest getting away for a bit. Even if you take a few days’ holiday, or manage to get out of the house and spend a good few hours a day away from them and on your own or with friends, it will do you a lot of good. As long as they are such a massive part of your life, you are always going to feel beholden to them and like a child. Start breaking away in small ways, even if you are not yet able or ready to do it in big ways. Work up to having more independence.
As for wishing your friend ill and resenting her marriage and happiness, the best cure for this is to start creating happiness for yourself and focus on that. Get out of the habit of comparing your life to those of others because there are so many problems with doing this.
Adopt a pet or take up a hobby or even just find something, anything, that brings you joy. Frankly, stay away from social media if you want my advice. Countless studies have linked excessive social media usage to depression and negative mental health outcomes. It’s poison for someone in your frame of mind.
Start small, and build on small achievements and small steps towards happiness. Good luck to you x