In love with a guy who has a girlfriend.(very long text,sorry)

Before I start, I want you to be completely honest with me and tell me if I am horrible for liking this guy. Please also know that I feel guilty, and I would never do anything that would hurt their relationship, nor would I ever try anything with this guy. Also, I'm 17 and have never been in a relationship before, so I might just be immature or inexperienced or smt. This text is also pretty long, so I'm sorry, and please ignore any grammar mistakes I make because English isn't my first language.

So, there is this guy whom I have known since kindergarten, and from 5th to 11th grade, we had also been in the same class. Despite knowing each other for such a long time, we have never really been friends, but we always got along very well. He was always really nice to me, and I have always liked him a lot. He, in general, is the kind of person who is nice to pretty much everyone and tries to help everyone. I could list at least 10 examples where he has been super nice and understanding to me in the last few years, more so than all my other classmates, apart from my friends, of course. He also has a girlfriend, and they have been together for about two years, I think.

At some point last school year, in 11th grade (I live in Germany), we had a kinda personal assignment for English class, and we were supposed to read it to the class. He wrote about his sister, who has a disability, and it was super sweet, and it made me realize even more what a great person he is. I don't know how to describe it, but it kind of made me think more about him. I also have to mention that I wasn't doing very well back then because of some mental health problems and stress, and I didn't have much motivation to go to school. That's why the thought of having a crush sounded nice, because it would motivate me to go to school, but it wouldn't mean anything else. I know that might sound strange, but I hope it kind of makes sense. A few days later we had this short conversation in which he said something sweet to me, and I don't exactly know how, but from then on I started to develop a little crush on him. On the one hand, I tried to fight it because I knew that he has a girlfriend (she switched schools after 10th grade), and having a crush on someone who is already in a relationship is terrible, but on the other hand, I told myself that it's just a little crush, that I'm just bored, and that I don't want a relationship anyways, and I'll be over this in a month. It was nice for a while because I was more motivated to go to school, because I knew I would see him (we had almost every course together, and we were in the same class). I was super happy when he would talk to me or in general pay attention to me. It was really easy to like him because he is really nice, smart, and good-looking, and even though we don't have a ton of interaction, when we did he was super nice and honestly more thoughtful, considerate, and sweet than most people. The problem was that my feelings didn't go away but just grew stronger, and I couldn't stop thinking about him. My friends started to tell me that it wasn't just a little crush anymore, and it was true because I actually really, really started to like him. Suddenly, for the first time, I actually wanted a relationship, which is unusual for me because I'm more introverted and I never wanted a relationship before, even though I had one serious crush before. So, at the end of the school year, I think I had really fallen in love with him, which is awful because he has a girlfriend.

Now it's a new school year, and I barely see him anymore. I constantly feel awful because I honestly never liked anyone this much. I can barely think about anything else but him, and I keep replaying all the interactions we had the last years. Like, I honestly feel so awful all the time, and I can barely function normally anymore. I also can't imagine ever liking anyone else this much, though I know that's probably not true. When I actually see him and he doesn't notice me (which happens more frequently now than it used to), I can barely keep myself from crying, but not seeing him also feels awful. But I know that I can't actually expect anything else because he has a girlfriend, and I'm awful for feeling like this for someone who is already in a relationship. No matter how much I like him, I would never try to come between him and his girlfriend, and I wouldn't have a chance anyways. I know I'm awful for ever starting to like him, but now I honestly don't know how to get over him. My friends keep telling me that it's super unhealthy for me how much I like him and that I need to get over him, but like I said, I'm trying, but I just don't know how. Maybe I'm just dramatic or smt, but I swear I like him so much, like it feels like barely anything else matters anymore. I think I'm actually starting to hate myself for liking him, but also because he doesn't like me and never will, so it feels like I'm not pretty enough or smart enough or just in general not good enough. I know it's stupid to feel like this because of a guy, especially for one who has a girlfriend, but I just can't get over him, and I can't stop feeling sad and empty.

Does anyone have any tips on how to get over someone you have no chance with but you seriously like? Also, I'm sorry this text is so long, and please believe me that I feel really bad and guilty for this whole thing.