I read an interesting article by a woman who wrote about her life as a child in a religious community that she now calls a "cult," though I believe it is one that if you were not a member and heard the name, you would just think it is a very fundamentalist church. (In other words, it didn't have an alarming name like "Church of the Charismatic Leader Who Harms People" or anything, but at least some of the people who know about it feel it is cruel to its adherents.) She wrote about learning when a speaker came to her school when she was about 12 who described abuse so the kids would understand the topic, that what was happening to her was pretty much textbook abuse. And she always, always regretted that she didn't ask the speaker for help, because everything the person described had happened to her in her church in the name of their religion. (She didn't raise her hand because she was afraid that if anyone even believed her, she would be taken from her family or they would get in trouble.) But she said living with it with no support messed up her life. She said that if anyone, when she was a child, would have just taken her aside and said she did not have to believe what her family was saying to her in the name of religion, that would have helped her to bear up and have hope when it was happening.
So I am saying to you, what your grandfather did was very wrong, and if he had not been a family member and it had been witnessed by someone who had no interest in keeping it a secret, he could have been arrested, especially for dragging you across the pavement. And, you do not have to believe it when they say you have to comply with this or you will be in trouble with your religion. The basis of all Christian religion is loving one's neighbor as one loves oneself, and the cruelties that are promulgated in the name of some religions, especially to kids who don't have many ways to protect themselves, are really wrong.
That said, you are still a minor and living under your parents' roof. All you can probably do is keep a very low profile, avoid setting off anything that would attract your grandpa to come at you again, and make your plans to move out as soon as you can take care of yourself. I know that is a heavy load when you have been diagnosed with the mental issues that you have been diagnosed with, but it can also give you some powerful motivation to do well in school and find a study path that will get you to a job where you can pay for yourself as early as possible. Until you can legally leave home, that is all you probably can do. (Of course, if your grandfather harms you again, you go straight to the police.)
I am sorry you are not in a good place but I will say, it is possible to rise above the pains of one's childhood when you leave your childhood life. Try to be brave, and be smart, and make plans to take care of yourself as much as you can.
I'm really sorry this has happened to you. It sounds very traumatic! I suppose your mum, having been raised by him, thinks your grandfather's actions are normal. They aren't. Luckily, it does seem like your father understands and is sympathetic. Is there any way you can live with him? If not, make it clear to your mother that you will not see your grandfather again. He mistreated you and you have every right to keep away from him. If she won't accept that, you'll need to reach out and get help. Call the police or talk to a trusted adult at your school. Let them know what happened and that you're terrified of it happening again. This sounds very much like religious abuse, and it's certainly emotional abuse to torment and trap a scared child.
It's really hard to get past the resentment and anger you feel toward abusers. It's harder in this case, because they won't apologise or acknowledge their wrongdoing. The important thing is to remember none of this was your fault. You weren't possessed, and you didn't deserve to be treated like this. If you had a real mental disorder, it was only exacerbated by this crazy maltreatment. So don't blame yourself and don't accept them when they blame you. The next step will be learning to accept that they may never say sorry for what they did. It's so painful to realise this, but it's important. The longer you search for love and kindness from them, the longer you keep yourself under their power. Unfortunately, you'll have to come to terms with this. Seek out friends and create a new family that showers you with love and support.