Tips on dealing with childhood trauma?

I grew up in a broken home and never felt safe as a child - both physically and emotionally. My earliest childhood memories consist of hearing constant screaming, arguing, and name-calling. A lot of times, I would hide under the bed and cover my ears because I didn't want to hear yelling at 3 o'clock in the morning. I think my grandma, who lived with me and my parents, must have some sort of mental illness that made her the cruel, selfish, and controlling person that she is. She never approved of my mom and made it her life mission to make my mom's life as miserable as possible, which then got projected onto me because I was my mom's son and my grandma thought we looked alike, so she often treated me with contempt and disgust as well.
It got to the point where my grandma would not allow my mom to even come near me because she was "incompetent and incapable of being a parent." My dad worked two jobs, so he was rarely around and when he was, his energy was mostly focused on all the fighting that was going on - the fights with my mom because she felt like she was being treated unfairly (which she was), and trying to calm my grandma down after one of her outbursts which happened a lot. I was stuck in the house, watching all of it happen, and feeling completely helpless.
My grandma went everywhere I went (including inside the school a lot of times). She barely ever just dropped me off at the door and picked me up after school. I was not allowed to ride the school bus because "what if the bus driver was on drugs" - literally. I wish I could make this stuff up. So, we walked to and from school together every day. I was not allowed to walk home with other kids because "what if we talked about things we should not be talking about." The walk home was basically time for interrogation about what happened at school, even if I didn't want to talk about it. I had two options - either I told her everything myself, or she would go inside the school, ask someone else, and completely embarrass me. One time, I couldn't sleep all night and failed a 3rd grade math test. My grandma was an engineer, so she wanted me to be good at math - and I was - but I was extremely tired, I was nervous because I was tired and I was worried I wouldn't do well, and I actually didn't. When we got back the tests, I ended up getting so upset that Mrs. Richardson had to call home and ask someone to come pick me up. My grandma came to school, used the back door instead of going through the main office, and came into my classroom even though parents were not allowed to. Not suspecting anything (because how could she), Mrs. Richardson told my grandma about the test. She assumed I was upset about failing the test, but it wasn't just that - sure, I felt bad about failing that test - but I was mostly scared about what came next. What came next was, my grandma and I got home, she took a leather belt and beat me with it to the point where I passed out. I couldn't come to school for a week after that.
On a regular basis, whenever I did something that she wasn't satisfied with - even getting a B when she expected an A - she would belittle me, call me stupid, retarded, and even tell me she wished I was never born. She would constantly shame me and tell me I was a burden, yet she never let anyone else spend time with me because she "knew best."
All throughout elementary school, I lived in the world of constant fear, doubt, shame, and never knowing what was going to happen next. She would also randomly show up to school during the day (again, that was not allowed, but she found ways) and watch what I was doing, where I was, and who I was with. If she didn't like who I was with, she would embarrass me and that person in front of everyone else. She was trying to pick my friends, but what ended up happening was, no one talked to me. All the teachers were okay with her constantly visiting because she was great at putting up a friendly and loving front, and no one knew what happened behind closed doors.
In middle school, things started to change. Rather than being scared every minute of the day, I found myself wanting to rebel. I met a girl named Katrina who was a year older than me, and she was known for being a troublemaker. Instead of steering clear, I found her cool and interesting. I started skipping school with her sometimes. Sure, I got in a lot of trouble if I was caught, but that was nothing new. I lied to my grandma about my grades - or I tried to, until she went through my backpack. I was tempted to break every rule - if I was going to be punished and berated, I might as well do something to earn it, right? Before long, none of the "good" kids wanted anything to do with me. My circle of friends was now Katrina and everyone she hung out with. I skipped classes. I disrespected teachers. I destroyed school property. I snuck out at night. I got into fights. I was a terrible influence.
When I was a freshman in high school, my parents moved to a different state, and my grandma stayed behind. One night, I almost got into a fight with a neighborhood kid. I thought he was going to reciprocate, but he never did. Instead, he stood there, staring right at me with a smile on his face. And then he said something I will never forget. He said, "Go ahead, Jaden. Go ahead and hit me. Feel the false sense of dominance from having power over someone who looks up to you." And in that moment, I just stopped. Not only that, I felt this overwhelming wave of shame. I remember just bursting into tears and telling Kevin I didn't want to hit him. And his only response was, "You can never lose by doing the right thing."
Freshman year ended up being the exact opposite of what I did in middle school. Kevin and I became good friends and rather than going against every rule the way I did for 3 years, I found myself scared to even slightly bend a rule. When I told Kevin I flet like I was going to fail my geography midterm, he said, "Then copy off me. Seriously. We all do it. Just don't make it a habit." I didn't. And now I wish I did.
And now, going into 10th grade, I feel so conflicted. I have no idea who I am. I feel so guilty about everything I did that I wasn't supposed to. But at the same time, I feel like I'm playing a game that I don't know the rules to. And several times each week, I have nightmares about everything my grandma did. I made the decision to forgive her, but I highly doubt I'll ever forget. Several people told me I was struggling with PTSD. Honestly, I don't even know at this point!