My supervisor does not believe my abilities
It's may not be in your best interests to continue working with this supervisor.  If you can find another supervisor, then it might be wise to switch, but you'll have to consider how changing dissertation topics might slow down your progress.  

Of course it's possible that you really are performing poorly.  If other faculty in the department are disinterested in supervising you, then that would be a strong indication that you aren't doing well in the program.