Greetings and peace be with you AnonymousGender;
I'm going mental and burning inside every day and night thinking they are together and I am suffering so much.
You have described anger very much like this Buddhist saying.
Anger is like picking up a burning coal with the intention of throwing it at the person you are angry with.
The person who gets burned the most is the one picking up the coal, the longer you hold onto your anger the hotter the coal becomes in your hand.
You stay awake at night plotting and getting more angry inside at the injustice, and it eats away at you, it makes you more bitter and twisted. All the time you hold this anger in your mind, the other person controls you, they control what is going on in your head, and you can’t do a thing about it.
The worst thing about anger is that the person you are angry with is probably oblivious of the harm they are causing you.
Forgiveness is not for the benefit of the other person, firstly it is to take away all the feelings of injustice and anger that burn away in your mind. It is a way of letting go, you do not have to pick up this coal to burn yourself.
You do not have to be angry, it is your choice
If you really must tell his current girlfriend that he has been going out with both of you at the same time, then do not do it when you are feeling angry. Like the prophet pbuh said if you are angry standing then sit down, if you are angry sitting then lay down. Do not go round to this woman for a showdown be as kind to her as you can, do not shout and accuse.
If you click on the following link there are many stories of people forgiving, murder, torture, rape, war crimes etc. The more stories you read the more you come to understand the power behind forgiveness. Spend an hour reading, it will be the best way you can spend an hour in your present state of mind.
http://www.theforgivenessproject.com/stories/eric-lomax
in the spirit of praying for the power to forgive.
Eric