Wow maybe you have to dicuss this with your doctor I was told today by my psychiatrist that there are many different medications in the same family we just have to find the right one. Weight gain is a common side effect, Ive also heard that when you are on medication it is hard to lose weight. Well good luck.
Its hard but it can be done I think,a good healthy diet,going to the gym or something. Some food supplements can help make it easier too.You could also ask your doc to try to swtich to something else thats less prone to cause weight gain.