hi again leonie,
i reacted badly to lamotrigine the first time too, got the rash day one. my epileptologist took me off for a few days until the rash cleared, then we started again on a half dose (12.5mgs 2x). it worked, within the first week i had 'traces' of the rash once but it was minimal and went away right away. if you had the rash too or something similar i'd suggest that before switching altogether.
in the end, seizure management is the main thing, so on the chance it may work for you, maybe cut the doseage right down to see if you still react badly. of course if you do, give it up, but i'm sorry to say now that you're diagnosed, playing with doseages is all part of it, happens all the time.
and the weight gain can be a toughie, b/c what happens to one person doesn't necessarily happen to the next. take tegretol for example, many on the forum have talked about it putting on weight, but it's never caused me to gain even one pound.