I feel for your situation as I've had similar things. My ex gf went one further and shagged one of my 'mates' (who I'd known since school and thought we go on pretty well with) behind my back. That 'mate' also works at my place, allthough not a manager it does annoy me as I've been made out to be the bastard who's made him do it etc.
He's a slimey twat and as long as the people I know and care for know the true side of the story I'm happy. I can understand your trouble at work as its very hard to avoid the urge just to smack the cunt stright out. I wont give him the satisfaction and just make his job as awkward as possible for my own amusement.
For your situation it sounds like your gf just does not know what she wants and as much as you like her you can't be with a women who is like that as they will always mess you about.
Dont get to cut up by women, they are just not worth it to be honest.
Start a fresh, get a new job, forget about the bird, bollocks to that twat of a manager and get on with your life, spend time with your mates, get pissed, party and go meet the many other nice ladies out there and use them for what they are there for.....a good shaggin