Always feel like crying. Behaviour!

I am in a relationship with this guy from 3 years our family know about us n literally agree we were planning fr an engagement or something when I caught a chat in his fon with a girl because of whom we were about to break up a year ago, I didn't liked this girl at all because of the way she talks to every guy,(she is so famous about her immoral behaviour regarding her character) we had a big dispute a year back n he swore on his mom that he won't ever get in touch WD her again (my guy he lives in Kolkata n I in new Delhi) but recently I came to know that they never stopped talking n are kind of dating since he came to my city 4-5 months ago (that girls lives in my city) now m confused in my life like anything, he keeps on saying that he didn't do anything wrong that can be called cheating nd z sorry fr wt he did but I feel cheated every now and then n m always crying m confused what to do I need help..