But you so readily believed those lies,,,,
Do you believe everything you are told? or just clichéd words from charming men?
You chose not to keep it in your pants HUWTT, you chose not to question his reluctance to leave, you chose to carry on sleeping with him whilst in the full knowledge that he was still with his GF.
Now why you would choose to act like this is anyone's guess, mine is that you loath yourself and you felt speshul being wooed by a taken man, in some way you deluded yourself into believing that his GF wasn't enough and you had that certain something she didn't.
Well now you know better, he was just a lying shit who wanted his cake, and got it quite easily from you.
Treat yourself better, you deserve to be happy, but you will not find it with someone who is already taken, and definitely not by telling the GF about your sordid affair.