The problem now is that this relationship is still intact while
you are having this dreadful feeling that you guy might be drifting
and you could lose this relationship. 
We shouldn't look at it from the preception on whether this
relationship is doomed to failure or not, rather the main issue is
what your boyfriend's definition of your relation, his inner
thoughts, and deepest feelings is.
Advice?
When a guy start to flirt around, BUT treats his gf good - it shows
cheap thrills that he is revelling in. But, where does you stand in
his heart - I can't conclude from looking at your words alone. You
HAVE to speak and talk to him about.
Don't start pointing fingers and trash things; speak in cool voice
and handle this matter rationally. When talking to him, appear
emotionless, neither anger nor worried, just be yourself like a
normal average day. He will be shocked and stunned when you talk to
him - do it in a way right, probably he will seek for your
forgiveness.
Remember your way of handling cannot be offensive in nature.
Communication is the key - Well, benefit of the doubt is given for
first time offender.
Then whats next? (Think Starhub 
)
You could try two ways; after the communication, the proper talk,
life continues and be happy with it - he would treat you better in
a way, at first.
OR, during the communication, ask him IF there is still anything he
is keeping away from you. If he were to say now, forgiveness will
be there. Warn him if one day, he were to be caught red-handed on
something - that's the end since it is multiple sins; Lying, breach
of trust, disloyal, etc.
P.S: Everyone should have a chance, but after the first one given,
its gone forever.
Cheers
