hi Ringy,
sorry to hear your story..
i can fully understand how you are feeling right now...
few months back, the same time happened to me too. My bf of many
years also betrayed my trust. Have been flirting behind back until
i found out. Unlike you, I made a big rage & i confronted him.
Though on the surface the thing seemed to be solved, but the
feeling that i experienced was exactly the same as yours. I totally
lost trust in him. I was shattered & started to wonder if what
i have done for him was insufficient.
problems started to surface & on many times, our relationship
was on the verge of breaking up.
i was miserable. 
an online friend~ here gave me his analysis & somehow showed me
a clearly picture of the whole thingy.
i tried to salvage by communicating with my bf only to realise that
nothing can really help as the mental & emotional trauma that i
was undergoing overwhelmed all advices given by people here &
my online friend...
until finally i seeked self-salvage. i asked myself what i realli
want. To remain or to leave? There was no way of pretence. if i
were to remain, i have to do forget.
but it was not at all an easy thing to do.
finally, one particular incident made me realised that my bf has
realised his mistake & he meant not to repeat it again &
that he loves me too. i still love my bf very much. so i told
myself that i will forget about it & start afresh.
N the good news is, I am finally able to walk out of the shadow
already. we are now starting afresh. Till now, everything is still
going on fine. But if things are to go wrong again, i am always
mentally prepared to leave without turning back already.
dunno if my online friend~ here manage to read this or not...(guess
you know who I am referring to) but still, would like to delicate a
million thks to him for stooding by me, to be always ready to
listen to me & give me advices. thks fren...~~~ 
ringy, hope by sharing this experience of mine, it can give you
some pointers.
my advice to you is to confront him. Don't suffer in silence. See
his reaction & judge for yourself if he really loves you.
pte msg me if you need further listening ear. 
