I'm up to 300 mg of Lamictal and feel great, but my weight is slowly increasing week by week. I'm very confused? How can I battle the weight gain?

Lamotrigene. Very few reported gaining weight, which could be related to being started on this medication. The Mayo Clinic notes used to treat bipolar disorder include lithium (Lithobid), valproic acid, divalproex sodium, carbamazepine and lamotrigine (Lamictal). All of these medications are known to increase the risk of weight gain except Lamotrigen. I would suggest to consult with a nutritionist and your MD.
Exercise. And check your diet. Are you REALLY the best foods you could be eating? You snacking on too many hollow calories? What's your alcohol consumption like? Battle weight best by exercising at 3-4x/week. In your case, you're young you should be easily able to go even a 4-5 day regimen. No need to go crazy but a good 30-60 min. of sweating in an aerobics, spinning, or other type of active lifestyle.