I believe this is a moral issue, and not mental health related. Unless bi-polar disorder causes you to do things against your own volition.
Yes, we can be more patient, loving & understanding......maybe even excuse some things people do if we discover they indeed have a mental problem. I agree that it might explain her flip-flopping, but this date with OM was "planned". She was counting down to the days till she could be with him. I think that is plenty cause for fresh pain. Yes, you had expectations, but who doesn't when they think they feel a sliver of hope in the air?
I am concerned about tomorrow. Maybe the two of you can take S3 where there are other things to help keep each other preoccupied. If you just depend on each other to carry the conversation, it may be tough.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!