Why is it a bad idea for me [21/M] to text her if I still can't stop thinking about her? [22/M]

My ex girlfriend and I started dating our senior year of high school, then another 2 years before she ended things extremely abruptly. She met someone that same week that she has been dating since she broke up with me 2 years ago. She knew me like no one else and I like to think I knew her better than anyone else too. She was my best friend and we would spend almost all of our time together and help each other run errands and shit. She was always way more head over heels for me that I was for her, and to some extent, I took her for granted. When she did break up with me, it was the most difficult couple of months I've ever endured, and she and I pretended like we were still friends even though there was a bit of flirting and a ton of me chasing her to facetime or anything to get attention from her. After a month or two, we got into more arguing and I basically told her I never wanted to hear her name again. We haven't had any contact since.

So it's been 2 years, I think about her virtually every day and every time I do, it hurts a lot. It hurts knowing she was so cold the way she ended it and it hurts knowing she's been with that other guy basically ever since. I know it would probably make things way worse, but lately the urge to contact her and maybe even tell her how I feel has gotten immensely strong. Help.

TL;DR: Ex gf of 3 years broke up with me 2 years ago. How do I get over it and why is it a bad idea to text her?