Lamictal,for me,didn't cause "medication induced" weight gain,in the past Depakote did,big time,while making a zombie out of me.
Lamictal hasn't done anything negative for me. My weight gain over the years are of my own fault/doing,not exercising,going on 50 y.o.,and eating a little more since I quit smoking last year,Iwas on Lamictal way before I had quit.
I once heard and expression,which makes sense,I believe the saying has been around for ages and appears in many books on Bipolar,maybe different wording but the gist is there.
"I'd rather be sane and heavy than skinny and nuts"
In a funny way,the latter isn't too bad if one can control and maintain their mood swings,but true Bipolar DXes require meds.
Lots of luck with this,I know it's hard,but you need to take care of the mind,and then the body.
Go over the meds with your pdoc,discuss the options/alternatives,your concerns. I always would tell my former pdoc, "how can one not be depressed if they are gaining weight and fatigued"? Her and I discussed the meds.
Also,have your thyroid function tested,and even re-tested, my GP's goofed up for years,causing me unecessary fatigue,hair loss,and depression. Hypothyroidism can have similar behaviours as Bipolar,but all from a different cause,that can be corrected with thyroid meds.,once done you'd be surprised how well a person with this can feel
For too many years I had been misdiagnosed with Depression,GPs handing out all AD's,ignoring my physical complaints,saying it was the Depression,bah,sonograms revealed Hasimotos thyroiditis.
That added with Bipolar can cause problems.
I suggest you take these sort of things up with the prescribing pdoc,see if he or she is open to rule out other things that can mimic some mental disorders.