Confused and a bit betrayed

I'm going through a lot right now with repressed feelings I've had for my ex I currently still live with. It's been 5 years since we broke up, and this year I was beginning to understand that more about myself, and work through my faults and trauma. I was going to start to talk to her and attempt to rekindle a romantic relationship . This was then turned into a negative when I found out she was talking to one of my best friends (well call him K). In these 5years me and her have had an interesting relationship. Its really upsetting because the past 5 years I was battling mental health issues and was constantly on edge and angry with the world. we fought a lot but, I always thought she still cared about me more than a friend. I always tried my best to show her how I felt through simple actions, such as going out of my way to make her comfortable, and support her through her struggles. To know that she would not consider my feelings in all of this, even simply as person involved in our small group of friends depressed me deeply. Not only that but they had been talking on and off for a year, even having a whole intimate relationship under my nose, meaning I had two of my closest people convering things up. All while I would talk to K about her about how sad I am that we cant be together. She had to have known that considering our relationships with eachother, one of the two of us would have to leave her life, and apparently she was prepared for that. I felt as though she didnt care about me at all, not even as a person. A lot of stuff was uncovered about K and she realized he was never serious to begin with, so now shes very upset, to my eyes it's more that they didnt work out, rather than she knows it hurt me so badly. It's hit me like an arrow to the heart, because I've known her for over half my life (since elementary school classmates) and I did and still do really love her. I still talk to her and even now I'm helping her through her pain of being lied to by K, putting her feelings before mine. I'm not sure if I want to forgive her but I still cant stand to see someone I care about even in the slightest distraught. I'm very lost