First off talk to your doctor and have a physical. Make sure your heart is in good condition for exercise. I am on lithium and it says not to change your salt intake without talking to your doctor, so make sure you do that. But salt makes you retain water, and lithium is salt, so ask about that. Also have a blood test to rule out diabetes (unless you already have it, of course.) Next, call your insurance company and see if they cover what is called Medical Nutrition Therapy (or MNT, if you have diabetes they may cover for this.) If they do, see if you can set up an appointment with a registered dietician. There are diet changes and exercise that you can do that will help.
There is a lot of evidance that certain foods are not only addicting but make you more hungry. Food such as fast food. The main thing you want to do is start looking at the foods you already eat and start with baby steps. When you buy bread look for a bread higher in fiber. Same with cereal. Add green vegetables to your diet. Instead of butter or margerine, choose healther fat alternatives such as nuts, (Peanut butter is good.) Slowly scale back your portion sizes instead of fully cutting out every single thing you eat. (This is why diets fail, because you feel deprived of things you want. Diet is both mental and physical.)
It is the same for exercise. Start slowly, but make sure you talk to your doctor first. Even a little bit of exercise every day is a start. And good luck to you!
I was only on lithium for a short time b/c I have to take a water pill daily for my heart conditon so I can't help much but possibly make a few sugguestions.
Have you spoken to your doctor and asked if their is another med that could help your situation and bp? I would stay away from Depakote (Valpuric Acid), it is nortorius for weight gain.
I hear it is better to eat several small meals rather tha 3 big meals daily. Can you switch your choices of food to fresh veg and fruit? Have you tried a high protien diet?