My crush on my best friend turned into a nightmare and I can’t escape it

I’ve been dealing with something that has destroyed me for a while now. I really liked my best friend. At first it was just friendship, but I started to genuinely like her and it turned into something way too heavy. I couldn’t stop thinking about her all day long. I felt a strong physical cold every time I thought about her or saw her, even when it was hot. I was anxious, constantly jealous, and sad for no apparent reason.
I asked her out a few times and she always gave evasive answers. I ended up confessing that I liked her and she didn’t feel the same way. After that I blocked her, unblocked her, tried to distance myself several times and always went back. I even went to the front of her house, stood there for more than forty minutes trying to build up the courage, and in the end I couldn’t say anything. At some points the suffering was so intense that I fell to the floor crying and even prayed, even though I’m an atheist, asking for help. The world lost its color. I felt like nothing else mattered. I got irritated with my family and couldn’t stay at home without thinking about her.
There was a period of more than a month when I greatly reduced contact and I improved. It seemed like it was passing. Recently she changed her look and I found her very attractive. I dreamed about her. The cold came back. I started feeling jealous again, feeling like trash, and getting sad without even knowing why. I began deliberately sitting next to her at school again.
Now I’m back in the middle of this shit. I’m scared of returning to the same nightmare as before. Has anyone else been through this? How do I make it stop once and for all?