I am happy that you asked it, but the situation/solution is much more complicated than you think it is.
Lets look at your situation from my perspective and this is important so that you can heal fully and enjoy good loving days again –
Why did he betray you at first place? The truth is unconciously you wanted to experience it. You would argue how this could be, but yes things are like that and the good news is if we take the full responsibility of our life’s experience, we can be the MASTERS and change things.
Forgiveness is really easy if you can empty your mind. You are having these emotions of anger, regret, internal conflict and confusion and if you can fully experience and express them you will be able to forgive him and you will start attracting great amounts of love, joy and happiness.
So a simple thing you could do to let go of anger is take a pillow and know its your ex, now start beating it, hitting it harder and faster and punishing it for doing such a thing to you. In less than 5 min. you will experience a calm state of mind which is forgiving and relaxing.
Similarly you can let go of all other emotions and you will be back in the world of love and happiness. 🙂