she did claim to have been suffering a nervous breakdown and I only have the other persons word that she's having an affair and has been unfaithful. Her behaviour has been extreme since we arrived here and certainly is out of character.
Cheaters will lie, deny, swear on anything under the sun to try to hide their affair, justify it, or hide the whole truth. You can't believe anything she's said or anything she tells you. I can't know for sure since I don't know her but I'd be VERY skeptical that she had a nervous breakdown.
And who was the other person who told you about her cheating during your entire relationship and are they someone you trust/believe? I'd take their claims with a grain of salt unless they have some proof or you have some reason/suspicion to believe they're telling the truth.
I also think you either need to kick her back out or lay down some serious ground rules. No drinking, no going to bars, no hanging out with ANYONE you don't know, open access to her cell phone, keylogger on her computer, and she has to let you know where she is at all times. If she doesn't want to do this or you catch her breaking any rules you set down kick her back out.
Don't make threats or give her ultimatums that you won't back up. Otherwise she'll lose even more respect for you (because she can't have much for you to begin with if she goes off with two guys who kick your ass) and won't take any future threats seriously.
To be blunt though I think you'd be better just kicking her out period and letting her work her way back into your good graces if she wants to badly enough. She has more to prove than you do. And you could always move on. If my significant other went off with the guy(s) who gave me a beatdown, there would be no second chances. I don't care how long we've been together. But that's just me.