I honestly think you need to just walk away. Your in a no win situation. The co worker treated you poorly and seemed to have no desire to work things out .. You tried and it brought your manager into the situation... Your Manager sounds like an idiot. So really you had no other choices.

The whole situation I think was rigged to get you out of there to be honest. This is why I think disclosing to an employer is just not/never a good idea.

I agree with Newtothis.. Write a letter get it all out of your system and then burn it.

I think you and your T need to work on this situation. I can tell its really bothering you and its not so easy to sweep it away and forget about it but , processing it through in Therapy will allow you get over the hurt,confusion,and betrayal ....

These type of situation are unfortunatly pretty common with people that deal with Bipolar..

Dont beat yourself up , right now its hurts like hell but, that will fade as you process it all and are looking forward to new probably better employment.