Why today is hard: remembering my ex on her birthday

Today is my so-called ex-girlfriend’s birthday so I guess this is the perfect occasion to finally tell my story.
We met when I was in 11th grade and now I’m in my 3rd year of college. Our relationship was mostly online but we had a lot of mutual friends. We started off as friends for about a year then slowly caught feelings and got into a relationship even though we never officially labeled it.
We fought pretty often but somehow we always managed to sort things out. We had our first major breakup more than six months later but we kept getting back together.
Things started changing when she began nursing college. Her behavior became really different. She started blaming me for almost everything and whenever I tried to confront her about things that were hurting me somehow the conversation would always turn back on me. I was constantly being called toxic an overthinker and even “the worst guy she had ever met.”
At one point she told me there was a guy in her friend circle who liked her. I told her that I was committed and didn’t have a problem with her having friends. A few days later she told me he was actually a very good friend of hers. When I asked about it she said he put a lot of effort into her and that she really appreciated it. I told her I could put in more effort too if she felt I wasn’t doing enough but she said that wasn’t the issue and accused me of being insecure and toxic.
After that she slowly stopped wanting to meet me and started ignoring my texts. Eventually we had a huge fight and broke up again. Our mutual friends tried to get us back on good terms and she told me she had no feelings for me anymore but that we could stay friends until things got better.
What really hurt was finding out that she had apparently been telling our mutual friends that I was the one who proposed to her and that she never actually liked me. The truth was the complete opposite. She was the one who proposed to me.
After everything I had seen and experienced I finally decided to walk away for good. It’s been about a year since then and today on her birthday I found out from mutual friends with proof that she had actually cheated on me with that same guy from her college.
Honestly what a way to find out.