GM SPECIAL: My Best Friend Is Dating My Ex-Boyfriend

I am so confused about this situation and I want to know if I am over reacting or this is very normal? I have been friends with my close friend for 6 years.



When I got into my relationship with my Ex, I introduced them together. Everything I went through in my relationship, both good and bad, she knew about it.

Long story short, I was with My Ex boyfriend for two and half years and we broke up... But after our breakup, He still wanted me back and my so called friend knew about this . She was one of those friends that said to me "Awwww, you two are so cute together, why don't you get back with him'

Well, I didn't take him back because I knew we were never going to work.(He is a big womanizer)
Even though we have been broken up for one and half years now, he only stopped making a move on me sometime in April 2013 after he surprised me at my so called best friends house with a cake.(In-fact, she planned the surprise with him). I was flattered and happy but sometimes, somethings that are broken cannot be fixed.
Fast-forward one month after that, I met a new guy and so far our relationship has been a blessing to me.
Then I started hearing rumors that my home-girl is dating my ex... I didn't believe it seeing that we only officially ended things 6 months ago.
Well, I walked in on them and dear Lord, You should have seen the look in her eyes.
And how she got busted was so funny...I went on a vacation in a different city with my current boyfriend and we went to a restaurant to eat one quiet morning...The waiter said to us that we have to wait 15 minutes and while we stood by the door waiting to be seated, Guess who walked in? My so called best-friend and my ex-boyfrend?
She couldn't even look me in the eyes...she was so ashamed. And to add insult to the injury, she was wearing the dress I bought her 3 months ago on a date with my ex....
6 months and BOOM, Besty is dating my ex boyfriend. I am honestly very hurt by this and NO, Its not because I want my ex back but I feel she broke the code and she broke my heart...

Even when She eventually asked to meet with me, she didn't even feel sorry for her actions but rather, she justified what she did...she said to me "If you are happy in your relationship, why should you care"....
I know the right thing to do is forgive her but I feel so much anger towards her? do you think I am over reacting? Please I want your advice because I want to do the right thing.