Hello Rumi
Sorry you are in such a place , but to give you my honest opinion , the beginning of your relationship just isnt right, he should be honest with his girlfriend and not stay with her and love someone else. In my country there is a saying that whoever cheats FOR you will cheat on you for someone else, so in my opinion I think you should just let it go because it is kind of cruel for the other girl and if he truly loved you he would have left the girl for you long ago , he seems as if he is keeping you on the side but deep down he still really loves her! Hope you make the right decision for yourself
Alpal