Why can't I forget her?

So the thing is, I had a girlfriend when I was 18. She was my first in any way, kiss, relationship, sex. After about 3 months, she broke up with me, and a month later she was with my best friend. A year and a half has passed, they broke up, and a few months later, we met at a friends house, at a party. We had some drinks, things went by and we ended up kissing on the balcony. We were together again, for another 3 months, then she broke up with me again. 3 months later she was with someone else again. I was heathbroken, she was the only good thing in my life in that time. Time went by, I met another girl. We were together for almost 2 years, then she broke up with me. And here is the funny part: I felt that she is gona break up, so i was prepared, But a week later I looked at the facebook of my first girlfriend, and I was on the floor. I am actually. It hurts more than the girl I recently lost, even if I loved her too. I don't understand my own feelings. I want to be with her again, but she is in a 2 year relationship. It's yust silly, i know, but I'm depressed for the last 2 weeks because of that, and I know if I tell this to my friends they just tell me to grow some balls or man up or I'm stupid or whatewer. I don't even know why I write it here, it's just good to share it with somone, who might understand. I search for her face everywhere. It's killing me. I'm a fkng 23 years old man with a stable job, runing a household, atending university, and this makes me feel like a 16 years old. I even have suicidal thoughts. Not like I gona kill myself, that's not a solution, but still...
And she seems unique. I never met someone like her. I tried tinder, and i deleted it, because I didn't want to lie to anyone. I can't be with someone else while a crave the kiss of someone else, I'm not that selfish. But the solitude makes it harder, and I don't have anyone to talk about this.

Sorry for my bad English, I'm from Hungary.
Why can't I forget her?
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