You haven't really confirmed whether you want this relationship to continue.
It is dying from a thousand small cuts and will drag you deeper into your depression if you let it.
You have to decide to make a decision. Either it is worth fighting for, or it isn't. If it is, your girlfriend must stop seeing her colleague and throw her full weight behind making you better. If it isn't, then let her go.....
But, please, please, please don't be *noble* about it all and send her off to another with your blessing.....your fragile self-esteem will wither away.