Im mentally destroyed

Im not well, i suffer for bipola-disorder and last tuesday i had my first mental breakdown in over two years i was so scared but i had nobody there i contacted my fiance but she had just finnished work and said she was going out with her colleuges we didnt talk after this really till monday when she confessed 3 things …
– She has sexual feelings towards a co-worker. And it he asked on the tuesday night she would have gone home with him.
– When down town she gets alot of attention and she likes it.
– She made out with her assistant manager on the tuesday night i had my meltdown.
I am really scared tbh ive never felt ao low i hadnt even started to pick myself up then this and she said ahe wanted to break up to live life . She stayed monday nightas she was gonna get her stuff , we chatted at point through the night i was a wreck tbh then boom tuesday morning she wants me doesnt want to loose me cant leave me and tbh seems to be really trying but i dont know how she came to that desicion and i think its cause she nos im dead inside i have nothing so doesnt want anything to happen to me , i dont know if shes filling me with a false sense of love between us or if she is been genuine. Where no longer engaged that was my disicion as she kissed him while wearing her ring so he knew she knew so ahe loat that privildge. But i cant tell if she is been sincer or is only there because i need someone and really im just holding her back because mentally im that messed up atm i cant even make a simple sentenxe when talking to somebody, i have never done this before but need advice from someone who has had the same feelings.

Thank you in advance