i feel for you man, i had a similar situation with ai lass i was dating, later engaged to, together for four years too ironically.
she started going out and spending more time with her family in the pub, when we were suposed to be saving for our wedding.
anyhow she left me one day, came with a couple of lads i didn't know to shift her stuff, etc.
anyways turned out she was shagging someone else, things were quite tense between us for a bit before she fucked off, should have guessed.

now 6 years on i'm with someone much better and life could be better, but that's nothing to do with us as a couple, we always laugh about things and we very rarely fall out cause i'm always right lol!

point: sometimes shit happens, pick yourself up and you'll find what you really want at some point, just enjoy yourself in the meantime.