How to make the right decision

[Mc-]

I need help on how to manage my thoughts on how my now girlfriend has just told me she has slept with one of my best mates during a period we weren't dating but were still keeping in contact. My beliefs have always been never to date someone a friend has slept with or to sleep with someone a friend has dated. 

But to only find out the information after i fell in love,  I'm torn between ending things now or making it work and going against my morals and beliefs. I feel anxious, betrayed, humiliated, and can't sleep or eat. I have already lost a close friend because i can't stand the sight of them anymore. I dont want to lose what i have with my girlfriend, but i dont know if these feelings and intrusive thoughts will go away. I want to know if there are any tools to help guide me back to a happy and clear mental state. I feel this is probably not worthy of support, but i dont know who else to ask. 

[Romminator]

This is tough. Friendships and love are powerful in both the source of our greatest happiness and can be, at times, our greatest miseries. I once had a debate with a close friend whether he would go out with the ex-girlfriend of a friend and he said no but I was open otherwise. I guess my stance was that now is more important than the past and if one's friend is truly happy for both of you to go out together despite having a past dalliance, then that is a supportive friend.
Mc- your girlfriend has chosen you now and what's happened in the past is, like they say, history. It's understandable that thoughts of jealousy, anxiety and betrayal rose with this new knowledge but maybe you can talk your feelings with your girlfriend and hopefully, she can reassure you that you're the one who's important now, you're the one that she wants rather than anyone else. 
Perhaps from a Zen or Stoic point of view, no one can really control anybody else. I think what's important is how she values, loves, accepts, supports, trusts, listens and wants to be with you now. 
Here's a quote from Marcus Aurelius (121 - 180AD): “Accept the things to which fate binds you, and love the people with whom fate brings you together, but do so with all your heart.” 

[Mc-]

Thanks Romminator, i appreciate your support. 

I have shared my feelings with her about this in depth.  She has assured me I'm the one for her,  in very similar words to yours.  I want to focus on the now and future with her. But am i disrespecting myself for continuing? Are these feelings going to go away or keep eating at me for the rest of my life wheather i stay or not?  We worked so hard to get to where we are now, so i don't want to make a rash or wrong decision that i regret.  But i don't want to string her along to find out i can't let this go. I'm not expecting anyone to make this decision for me, i know its mine and mine alone.  I'm just so confused. We have good times still, but the thoughts of them together and the betrayal keep popping into my head.  There's so many triggering words and things. Having happened in my friendship group I'm never going to be able to just completely shut this out.  I'm going to hear his name or see him and it will remind me of it all.  She is great friends with his sister,  i can't expect her to never see her friend again,  so i know she will end up in the same room as him and that will trigger me again. 

[Ggrand]

Hello Dear Mc,

A very warm and caring welcome to our forums…

My youngest son had a friendship group when he was younger, his best mate was dating one of the girls in this group for a number of months until they split up,  both remained in the friendship group….I can’t remember exactly how long it was, I think a few months after their break up, that my son started dating his best friends ex girlfriend…but still stayed and continued on with their group of friends…

Fast forward 2=years, my son and girlfriend got engaged and married 2 years later, my sons best mate (girlfriends x) was best man at their wedding and became Godfather to their son….All through the years my son, his wife has always remained best of friends with my sons best mate…My sons best mate eventually found a girlfriend who he married many years later and my son was best man at his wedding….There friendships have been ongoing now for over 30 years….and continues on strong even today..

I am not sure how to express myself but I think I want to say, that true love….is totally unconditional love, and it lives in the moment, not in the past…it begins at the time, you fall in love with her…and it grows stronger and more purer each day you spend time together and both move forward….What happened before your love for each other started to blossom is not worth a minute of your time to worry about…

Love is worth fighting for, she has been honest with you, because she must love you and wants to continue moving forward with you…Is your love unconditional love?….would you be considering breaking up with her, if it wasn’t your friend she slept with,  but an unknown person to you?….before you even started dating? ….Try hard to remember that the past is the past….you didn’t love her then…Is your love strong enough for her…to start your lives together from the first feelings of love you felt for each other…whatever happened before those feeling came into play, should not take away  your love for her….

Enjoy each other’s company, move forward, look towards the future with each other…(not the past)  love and care for her as much as she loves and cares for you…after all she has chosen you…

My kindest thoughts and best wishes..

Grandy.