Cycling is pretty efficient so you have to ride a lot to burn off a lot of calories. Try mixing in walking/jogging to help maintain your weight.

Also consider taking a cooking class if possible. It might make cooking and eating healthier a little easier. I've been told that the key is to have a lot of flavor in your foods so you don't have to eat as much to enjoy it.

You may also find that riding your bike a lot may help control the bipolar symptoms a little and maybe you can cut back a bit on your meds once you have things under control. I don't know if this will actually work, but I find when I ride a lot, I'm a much happier and calmer person day to day. Something to do with the endorphins from exercise.