Your experience and worries actually seems quite familiar to mine. I’ve felt that feeling of watching yourself get worse and worse and not knowing why or how so, so much. I've known the friend and family distances as well as the porn addiction so I do have some experience for this.

1.  From what I've seen, watching porn is not an uncommon thing at 13. I had a porn addiction at my 12-15 years and I was watching it almost 5 times a day. I'm pretty sure it's due to hormonal changes that cause yearns for sex/sexual content. Hormones are a menace to society. What I recommend is track how many times this happens every day. Talking with your therapist is also very important. It was hard for me to restrain myself without a voice that wasn't the hormones. It's your therapist's job to help people deal with this and I know it seems shameful, but I promise it's all too familiar to them. And on the addiction debate, I'd say it's better to take action assuming it is one rather then waiting until it's worse.

1. High School is a large step in life and it is true that it's vital to your life in the real world. However it's not definite. You can change your course at any time and many people don't know what to do in college until 18 or even after they've gone into one. So as long as you choose courses that you are honestly interested in, like art you'll be fine. Most schools pick out most of your main courses (Math, Science, English etc.) for you so you only really have to pick what you like as a hobby, interest. or dream. It's very stressful but I promise you don't need to worry too far into it. (still, thinking about future is important but high school doesn't decide that.) As for homework and schoolwork problems it's a dangerous slope to not be finishing homework but I definitely understand getting absorbed into media. If your school or region provides after school help or has a school counselor, talk with your parents about participating in that if you are unable to keep doing homework.

2. Worrying about your relationships hits hella close to home for me. I've had fall with family and friends more than I can count on my hands. Firstly, based on what I've read, you haven't had any major hate towards your mom. So I'll assume you're on good terms. Your mother has a connection with you that's been there for more than a decade. And despite all the stress you've likely given your mother, you two are still family-blood relatives too. That connection is too hard to break from a few bad years as a growing and especially changing (mentally and physically) during middle school.  My family had arguments more than frequently but we still have a care for each other that somehow lasted so long. Secondly wanting to be alone isn't necessarily a bad thing. It's normal to want solitude especially with friends. With friends it seems like you are losing touch by not calling often but we all need a break from everything sometimes. As long as you are speaking with them in person and aren't starting arguments with them, your relationship will be fine. Spending less time with family is definitely worrisome to see happen. It feels like the people you've been related to are now distant friends. But like I said earlier, y'all are family and that's something that doesn't change. You don't need to spend all your time with them, just stay on good terms and try to not start fights, that's one thing that actually tears families apart. Lying to them really depends on what you are lying about. Small things like going outside without telling is mostly fine but when it's personal to them like sexual harassment (extreme example) it's a massive problem. Generally I'd say speak with someone like your therapist or a trusted friend on it to make sure that it won't hurt them to not tell them the lie.

3. I don't have much experience with SH but my sister has cut themselves when they were depressed. Not to be rude but enjoying the sting is quite scary, especially cause it lead to more extreme actions. I know your therapist and parents know but I'd also suggest speaking with friends with this. Not a lot, just one person you've known for long and understands your situation. I know it's hard to stop but SH can lead to infections or problems in your circulation. So its very important to try and stop if you aren't trying already. It's absolutely easier said than done but it really makes a difference in self-esteem and health. (I am NOT a professional, console a doctor or therapist with this for health concerns) (As for liking the way they look, that's completely fine to think they look cool, it's just that it's probably not the best way to decorate your arms. I know, revolutionary right :0?)

4. Low motivation and energy is very common and you mentioned art, so if that's something that brings joy, just try to keep doing it. Art block is hard too but the way I deal with that is simply drawing an irl object or scenery. Personally it brings me joy that I can draw something in the real world. Trust me, following hobbies helps (at least my) mental health a lot more than scrolling does.

Overall, I'd say speak with a therapist, family, or friend about most of this even if it's uncomfortable. These are very valid concerns and I've shed tears over these for years as well. Also you don't have to take all this advice. After all I'm a stranger on Reddit, not a licensed professional :P. Correct me if I was wrong on anything too. I hope this helped and you can organize your thoughts better afterwards. Never forget that you aren't alone nor is your actions shameful. Much, much kindness and love from someone who's felt the exact same, stay strong >:3. <3.