my best friend and i kinda had a friendship break up a while ago

i had a best friend for about 4 years, and she was the most amazing human being ever. we shared everything, and did everything together. i always had trouble making friends, so she was basically the only person i would hang out with (and maybe some other people, but i never really got close to them). maybe i just have weird trust issues or something but i don't know. anyways, our friendship ended. i have literally no idea why. isn't wasn't just one day where we just stopped talking and that was it, but it was just a slow and gradual process where it hurt more and more every time i saw her. maybe i sound obsessed or something, and maybe i was. i really don't know. we go to the same school and still see each other a lot, but now i can't even make eye contact with her without feeling something bad that i can't explain. i understand that people gradually shift apart, but there's always a reason, right? like you just don't feel the same passion as you did when you first met, or their personality changed... but in my eyes we're both still the same people. i just want closure. it ended so slowly but it felt so fast that i couldn't even get the chance to process what was happening. she's not dead or anything but it kind of feels like it. i really miss her. i have some new friends but it feels so weird hanging out with them. i want to text her, or talk to her, but now it just feels like we're complete strangers to each other. i think it's too late to rekindle our friendship, but i miss feeling those days of just happiness. i mean, i was depressed before we became friends but she really helped me feel better, maybe that's why i miss her so much. again, i really just don't know anything, and i want to know why this happened. maybe i did do something and just haven't realized it so i'm constantly living with this unreasonable anxiety and guilt that i just want to go away. idk what the point to this post is but thanks for reading this far if you did.