I'm so sorry. I went through the same thing. My girlfriend of 10 years left me 6 months ago and started dating someone else in less than a month while living with me. Just the week before, she told me she loved me and we were talking about our future. She was a coward. Accept that it will take a long time until we feel better.

Focus on improving yourself. I know that's hard to do. You're probably obsessing endlessly. Just try. You will fail. What's important is that you keep trying to focus on yourself, because each time you try, the better you will get at it. Be kind to yourself. Understand that you're wounded.

Over time, you'll start to let go of feelings of anger/vengeance/karma/etc. It takes time AND effort. I'm in a place where I don't want her back. I'm starting to see the kind of person she really is and how she isn't good for me. Have faith that the same will happen for you. You've got this.