A person taking psychiatric medications whose side effect is weight gain for the most part gains weight NOT because of eating more. There are medications that cause weight gain and appetite increase, and the weight gain is both directly caused by the medication and indirectly added to via eating more, but those are in the minority. Most psychiatric medications that cause weight gain do so via many different pathways and you can be eating 500 calories a day and still gain weight on them.

OP - a lot of people have gained weight on lithium and a lot have not; the fact that YOU gained weight on it is indicative of YOUR propensity to gain weight on it, so you might gain weight again if you go on Lithium. However, since you were in HS, maybe taking Lithium coincided with a growth spurt and some of the weight gain was actually normal. Do you remember?