If she is harassing you at work or upsetting you then you have grounds of bullying and your employer has to take action. Is there anyone else at work who could back up the problems. If your boss can't help you another idea would be to contact the citizen advice because if you do have to leave your employment due to ill health you can sue your employer for constructive dismissal.
My niece works in a private care home she had a few problems with her work two years ago, she took seriously ill and was of work for almost a year. The boss of the care home didn't want to pay her any sick money after the first three four months and he didn't believe she was as sick as she was, he even suggested that my niece just leave because he didn't think she would be able to return to work and he wasn't given her any more money. My niece got a lawyer and was in the process of taking him to court. After six months her boss backed down and paid her the six months backdated sick pay, she eventually returned to work.
Sarah try and get this resloved if your employer is trying to brush it under the carpet then go and get some advice for ACAS, don't let it fester and affect your life. You shouldn't have to change jobs just because of a stupid bitch.