Why do I feel so deeply unhappy when I have everything?

i’m a 15 year old girl and around two years ago, my life was amazing. I have friends and really cool school. I also live in different place with changing weather. I always felt happy and calm like when you look back at nostalgia but the nostalgia your feeling is at that moment. but now I live in a different place away from my friends and I live in a high-rise, which is the way I’ve always wanted to do, but I don’t feel happy because it’s not the way I want it. For example, my one of you is just a building because I would get any sunlight, but before that we lived in Airbnb and I had sunlight I didn’t have my own room and I thought if I got my own room, I would be happy but now I have my room and I’m not happy. I also don’t go to school because the school. I actually felt happy and was like a hippie school and I can’t find a school like that where I am now also I don’t have friends. I like I have friends, but they’re my mom’s friends children and they’re cool but I want friends that I really wanna hang out with? Also really big huge issue is that we were supposed to go to a round trip in Europe but my mom can’t do that because of her job and I was so excited and it just never happened and we never talked about it again. and makes me really sad. so we just live in another country where she can do her work and it’s a country. I can’t speak the language and my mom can speak the language, but since she didn’t teach me how to speak it I don’t know how to interact with people I went to school here for a little while, but I didn’t like it because I can’t speak the language and I’ve never done good in traditional school. Also, my sister’s here I don’t really like my sister because she’s just such a bitch, but my mom expects me to talk to her, but I don’t want to and at the moment I’m homeschooled and I mostly just sit in my room all day because there’s nothing for me to do but I know better situation and then going to regular school cause I was miserable there, but I’m miserable here too, and when I first came to this country I wasn’t miserable. I was actually quite happy in the nostalgia feeling. I was describing what you’re feeling in the moment I had that feeling, but when I came back, it was my sister because before I was just with me and my mom, but then we had to get my sister and now I’m just not happy I know I should be happy because I have everything a housekeeper or high-rise food money but I’m just not happy the reason I went out of my school is because I couldn’t take anymore and I threatened to do something to myself so me and my mom went to a therapist and I felt such relief at the time but not for long because I new I was going to experience more loneliness and I’ve asked my mom if I can go to her again, but sometimes when I ask her for really special thing she kind of forget she’s neglect anything but she just has a lot on her plate and I don’t wanna stress her out. What should I do? I do have hope that when I get older, it’ll be better but I’m not really sure please help.