Venting is fine, to a point. Give yourself a chance to do that for awhile, but don't get stuck there.
I have no motivation and even if I pass I won't get good grades. Practically and in learning situation I'm about as intelligent as a piece of bread.
That right there is a big obstacle!
ADHD....... My T has it. I haven't been officially diagnosed with it but he says I, at the very least, have "ADHDish tendencies". But, in my experience, just dealing with all the PTSD/family related stuff you've got going on in the background can make it pretty hard to concentrate too. There have been a lot of days lately when it's felt like I'd be better off if I'd never started therapy and stirred all this up. That's not actually true, but it feels that way.
I can see where it would be hard for the people around you, who've never experienced what you're experiencing, to not totally get it. And there ARE no easy answers, so it's tempting to just say dumb stuff like "think positive!" (I used to respond to that with "I'm positive I can't do it!" LOL I don't recommend that. It never worked well for me.) I hated school too. Thought of it as "serving my time". But, lucky for me, I love to learn and still do, in spite of "them".
So, I've bee trying to think of what helped me that might help you. When I was about your age, I read in a book that the job of every human being is to chose a teacher and find a friend. And that turns out to be good advice. Not easy to do, but well worth the effort. (I didn't find a friend until much later.) But I did find a couple people who were willing and able to be mentors. Very valuable! And I found a thing or 2 I could be passionate about. Didn't have anything to do with school. Stuff that meant something to ME that I could apply myself to 1000% when I wanted, the way I wanted, with no need for judgement. (I did and still do, have a tendency to be pretty critical of my own efforts. Avoid that as best you can at least with one thing.)
You're smart. You DO have a future ahead of you, even though it doesn't feel that way. (That's a symptom, not reality.) Don't burn any bridges you might someday want to use.
Here, homeschooling is an option and they have programs for people with "alternative learning styles". Some of that might help you, but if it isn't available, it isn't. How much does your school know about your situation? Have they given you any specifics on what "accommodations" means to them?