Lamictal is a great medicatio. It's worked marvelously for me - no other med did, ever.
I have an ED, too, and was bulimic for 16 years, now just very low weight/anorexia.
I know about that fear of gaining. For me, though, I"ve lost a little on the Lamictal (I can't afford this and hate it) because it is energizing and made me move around all the more.
It's weight-neutral, as least for me.
Glad to have you here!