I've been on Lamictal for a few years, it doesn't cause weight gain, it's really a good mood stabilizer.
It may be a good idea, if you already haven't, to have some bloodwork for thyroid functions and if possible a thyroid ultrasound, some of this can be coming from an imbalance or thyroid nodules.
Lamictal as a mood stabilizer, I've found to be good, I didn't even have to include AD's with treatment, just the Lamictal. Not everyone experiences the same thing with meds., some benefit, while some do not agree with one's body or mind, so it takes trying a few and/or combos.
Depakote was not good for me, it was zombifing,constant hunger (caused weight gain) and constant fatigue, switching to Lamictal was the best move for me.
Lots of luck with this.