I believe Lamictal is used to treat PTSD as well as the other uses you stated. but you could speak to your psychiatrist more about this.
Lamotrogine is Lamictal and has been a miracle medication for me. I have Bipolar and occasional seizures brought on by stress, I also have PTSD. I have only ever heard one bad report of it and i know many many folk on it from a chat room for mood disorders
With meds you definitely have to do your own homework as well as listen to the professionals you see. The recent revelations about seroquel marketing are just another reminder to us that often meds are heavily marketed for purposes that have no FDA approval and no studies to back up either safety or efficacy. Which doesn't mean they aren't perfectly appropriate and the best alternative, it only means that professionals often recommend what they've heard the most about lately and that usually comes from the manufacturer trying to maximize income. It is perfectly legal for a physician, once a med has been approved by the FDA, to use it for off-label purposes. What isn't legal is for the manufacturer to talk the physician into using it for off-label purposes. So this med might be the best thing for you, and it might be completely inappropriate depending on where the professional got the info from. So you're right to question things, but as I said, that doesn't mean this isn't a good med for you. The biggest question we all face is whether we need meds at all or whether therapy alone will do the trick, since none of them will cure us and they all have downsides, but if you need them, the best you can do is see people you trust, research the med on the internet as much as you can, and then make your decision. Good luck however it turns out.
I know someone with Tricotillomania who was helped by Lamictal. He does not speak and has an intellectual disability so it is hard to acurately diagnose other possible conditions. Hos mood definitely improved.
Sometimes the best way to find out is to try and see what results you get. None of this is an accurate science.
Good luck to you.
I just found this in wikipedia: Other usesOff-label uses include the treatment of peripheral neuropathy, trigeminal neuralgia, cluster headaches, migraines, and reducing neuropathic pain.  Off-label psychiatric usage includes the treatment of depersonalization disorder, bipolar II disorder and other bipolar disorders, schizoaffective disorder, borderline personality disorder and post-traumatic stress disorder. Lamotrigine has been studied as an adjunctive therapy for treatment of refractory unipolar depression, attaining efficacy on the secondary metric for treatment outcomes (Clinical Global Impressions), but not the primary metrics (Montgomery-Åsberg Depression Rating Scale and Hamilton Rating Scale for Depression).