My husband is currently just on Lithium but has been for 10 years & in the past at different times been on different antidepressants & had gained weight too. Last winter he went on the South Beach Diet much to his doctors dismay. They monitored his med levels & him closely. He had great success & lost 22lbs. and since kept it off by watching his carb intake. His doctor found it didn't affect his levels at the time & losing the weight helped him feel better about himself. Like you said, everyone is different but I hope this info helps you.
Sooblue
hi Sue,
I was on lithium at one point for around two months, and I had really bad side effects. the main ones I noticed were weight gain and severe drowsiness. that's when my doctor switched me to lamictal, which really had no side effects for me and I was able to lose most of the weight I gained (5-10 pounds). I think in your med combination, lithium might be the culprit for the weight gain.
Michelle
Kittycat is right about the Zoloft - that shouldn't be your cultprit, but lith does cause weight gain in almost everyone. Zoloft really shouldn't be doing that. You say wellbutrin doesn't do anything, but have you tried adding it to the Lamictal? That's usually the trick for most BPs, 3-400mg of wellbutrin with 200 of Lamictal as a combination.
As far as "diet"s 000000 - I hate that word 
Just eat SENSIBLY, quit eating the junk food, cook dinner with fresh foods, and EXERCISE. You will NOT lose any weight without exercise no matter what "diet" or starvation tactic you use. I had to lose 30 pounds after being on Depakote and Zyprexa combination, but I did it and now I look really good. You are so right, being overweight in and of itself is a big reason to be depressed, I was miserable because I had always been underweight. I actually gained 55 pounds in 3 months. I lost 30 so now at my perfect weight. It CAN be done if you try. If you do not have a Weight Watchers near you (which I highly recommend), go to Jenny Craig, they will teach you HOW to eat and their food is good. It's expensive, but if you exercise at least 4 times a week for 30 minutes it will cut your time in half. (I was a personal trainer/nutritional counselor for a time before I was a nurse, so if I can help, just ask).
Shannon
Variety is the spice of life, BP is the key
I've been on Lamictal for 5 years for seizure disorder. This medication has no effect on my weight. 6 weeks ago I went on Zoloft 50mg a day for bi-polar. before starting Zoloft, I weighed 145 (i'm 5 6 and big boned) and since then went down to 134, so far. I've read on drug websites including the Zoloft website that weight loss is a side effect. However, when my neurologist put me on Zoloft, he told me It would make me feel better but, I could gain 30 pounds. To be aware of this. He said that weight gain is more likely than weight loss, and the drug companies don't tell you this because of the competition. The reason he told me this is because every patient of his has gained an average of 30 pounds. He also told me that he believes Zoloft is the best medication for Bi-polar, but, if weight gain is an issue to change meds ASAP. Hope this helps
Kittycat you are soooo right - I hate when docs write these meds! Ahhhhhhhh! They are fine and dandy for clinical depression, I'm glad those people can have them. They almost always cause hypomania, I just learned recently from the doc that frequents this forum, whom I respect immensely, that now there is a warning for doctors to screen for bipolar before prescribing it, because can be DANGEROUS. 
Our brain chemistry is completely different from other people, what works for other people usually affects us differently or opposite. We get wired at the drop of a hat. Then, you have to come down eventually and the depression is even worse. Nice drug. I've been on them, too and can attest to thier bad effects, not to mention how much I have seen! Why can't they just write the preferred, Lamictal and/or wellbutrin for BP?? 
Works much better, much safer for us. Of course, tooo much wellbutrin can cause mania, but it's not common at all on reasonable doses.
Ok, off my soapbox! Thanks for listening. 
Variety is the spice of life, BP is the key
Variety is the spice of life, BP is the key
Regarding these meds I should add, they are working great for now, but, I haven't been fully tested. Meaning what turns me into the manic stage is sometimes triggered by idiotic, annoying people I work with
And out of fear, I've stayed away from these people. They fear me too because they think I'm crazy and someday will end of going postal.... Because of my mood swings. actualy they think I'm on Meth most of the time. When I'm away from these people, I'm more on the deppresive side, extremely suicidal and all the other symptoms of deppression. And those feelings are under control, I actualy feel normal, how weird. I haven't gone up.... or down.As far as weight gain/loss. I wonder whether some of the symptoms are because you feel better and therefore work harder on losing weight or have an increased appetite from feeling better causing weight gain. In my case, I used to eat chocolate everyday, ice cream, pies, french fries, all that good stuff, but, now, my appetite seems to be less. I know one thing, since taking Zoloft my digestive system has slowed down tremendously. a few prunes and an apple a day took care of this problem. So maybe it's a combination?
I'm sure it's because of the change in mood. Many depressed people have no appetite, or too much of an appetite, then when they feel better, they either eat more or less. Mood has a LOT to do with appetite; SSRIs aren't known for weight gain, actually prozac was being prescribed for appetite reduction, not sure if that was an actual indication from the manufacturer tho.
The old ADs, tricyclics like Deseryel (trazadone), Elavil, Tofranil, Anafranil, etc. caused a voracious appetite for sweets, so people gained a lot of weight on them, similarly to Zyprexa and Risperdal, two of the atypical antipsychotics. Lithium and Depakote cause weight gain, too. I am lucky enough to have found a combination of drugs that do not affect weight, I can't deal with that! I'm a fitness and health buff, so my 55 pound weight gain in the first few months on zyprexa and depakote were not too pleasant! Took a lot of work to lose it!
But like I said, all these meds affect everybody differently, what increases appetite in one decreases it in another, it's crazy!
I wonder if your tumor has some bearing on your moods? Where is it located?
Variety is the spice of life, BP is the key
I agree about SSRI's not having an affect on weight because everyone I know who has taken these meds are different shapes and sizes. The only side effect they said they had was sexual side effects, but not weight. My doctor told me I could gain 30 pounds on Zoloft because all his patients have, I think he says that to cover his butt. But, I think everyone is so different that it's impossible to come to any conclusion. And some people who have weight problems aren't entirely honest about trying to lose weight.
My sister is a perfect example, she was a normal weight before taking zoloft (for a year) after going off zoloft she gained 120 pounds from boredom and deppression. She kept telling me her metabolism was really slow and she eats normal but, can't lose weight (no matter how little she eats). What she wasn't telling me was the extra calories she was consuming from chocolate. 2 Years ago she had gastric bypass surgery and lost all the weight and hasn't gained any back.
In my case I was so terrified about gaining weight that I changed my eating habits ( not eating less than before Zoloft), but healthier. So far it seems to be working.