Not every kid gets the parents they deserve. Sadly, you are among them. You need more help than I can reasonable supply in an advice column. Please look into whether there is an Alateen chapter in your area. Look online for more information about the organization. If there aren’t meetings locally, consider joining their chat. Also consider joining one of the forums available here at Psych Central to get more support.
Regarding the issues with the “almost boyfriend”: It sounds to me like your parents’ reactions were more about control than about the boy. Ditto regarding the use of the truck and when you got home. If that’s the case, I’m afraid you’ll have to figure out how to navigate their need for control with your need for independence. Please consider getting some therapy to help you determine how to manage it. Fortunately, at 18 you are almost able to get out on your own.
You need to be making plans for leaving the situation you are in. It’s not going to get better unless and until your parents take their own recoveries seriously. Fighting with them won’t help. It will just make you feel worse. Talking to them calmly and with caring might help but, even if they do go into a recovery program of some kind, change will take a long time. Complicating things even more is whatever effect the brain injury had on your mother. I hope she is getting the help she needs to recover as much as she can.
Talk to your school guidance counselor about what you need to do to get into college (and to pay for it). If you’re not ready for college, look into gap year programs. Not ready for that either? Plan for how you will be able to afford to get a place of your own (probably with housemates) when you graduate. If you haven’t already, get an internship or a part-time job that will give you some skills and a resume.
Please, please don’t make the mistake of moving in with a new boyfriend to get out of your parents’ home. That is not a solution that will end well. You described him as your “almost” boyfriend. That’s not enough of a relationship to move in together. If the relationship fails, not only will you feel awful but your parents will see it as justification for their concern and control.
I wish you well.