Hi Rosie, you need to maintain a healthy weight to stay fit. You don't say how tall you are, whether you are small or big framed, but 6 stone 10 sound very light. You haven't been diagnosed for very long and you may be suffering from depression, it is a lot to take in to start with and you are having to adapt to a whole new lifestyle. You are aware that you are being illogical and irrational and that is the first step to recovery. Please go and see your GP and tell him how your are feeling.
Good luck and let us know how you get on.
Marilyn