there's 2 scenario.
a) u found it out urself;
b) she told u;
either ways, forgive but not forget.
relationships should be based on trust.
if he/she crossed the line by doing such a thing behind ur back,
cry ur heart out and forget abt her. dun have to find excuses for
him/her. dun harbour hopes of him/her coming back to u. tat will
only make things more difficult for urself.
personally, i'll never trust this person with my heart again.