Infidelity Support Group
Any relationship in which one partner engages willfully in sexual relations with another outside of the partnership is considered to have experienced infidelity. This breach of trust is often traumatizing for the faithful partner as well as the relationship, and support is often needed to heal emotionally and to decide whether or not the relationship should continue after...
There is no satisfying answer to the question "why" and "how could she". It comes down to that cheaters ate selfish. They only think about their own needs. They lack empathy and morals and are incapable of loving anyone but themself.
She has had a full blown affair. She left you to be witb him. As much as I can guarantee you her new relationship won't last long as they never do. You sure don't want to be her plan B.
You can't change her you can't fix a relationship if she's not committed.
At this point there's nothing else you can do for your relationship. Best you can do now is get therapy. Infidelity is vety traumatizing. Therapy is vety helpful. And try to get out of the house hang out with friends do things to distract you and make you happy. Focus on you.
It seems she hasn't developed in a mature person who is capable of love or a healthy relationship.
You are still fortunate that you didn't get married and therefore can move on without too much entanglement.
See a counselor and heal before starting your next relationship. Try to find someone with a strong character with good self esteem.
Try to get some individual counseling for yourself to help you through this and more importantly to learn how to "fix your picker" so that you see these kinds of people for who they are before entering a relationship with them. If (most likely when...) she comes crawling back, do not take the bait! Go no contact, do not engage on any level with someone who would do this to you. She will lie to you all over again; I guarantee it.
One important thing is to accept that it takes a while to process all your thoughts and heal.
Slowly you will give up the illusion of the person you thought she was and realize who she really was all along.
You also need to grieve the loss given this was a long time GF.
Also take time to think about what kind of characteristics you want in a girlfriend and choose more wisely in the future. Look at character first.
I suggest someone with healthy self esteem, mental/emotional healthy, and is honest and open. Stay away from girls who hang out with a lot of guys.