What was best friend treats me really bad?

I would not cut- off all communications because you already know how that scenario will probably play out. Just be friendly as needed to work together. Quit putting forth the effort sounds like it is one sided as is and when you do, you get burnt. Make her pursue the relationship, don't ever come across needy and show her that you can get on with or without her. Friends come and go in life for what ever reasons. Keep your friends close and your enemies even closer. She doesn't seem to appreciate you or respect you. Not what I would consider a best friend. Be casual and leave it at that your co-workers have eyes and knew you first so if she is the type to stir up drama they will realize that soon enough.