Gaining weight is not a petty worry. I have gained from Seroquel and my blood pressure, blood glucose, and cholesterol have all risen.
From me, who has been on many antipsychotics, be cautious about them. They really change your life, and not all in good ways. If you do use an AP keep it at a very low dose. You don't have psychosis, do you?
For BPD it seems that an antidepressant and a mood stabilizer (such as Lamictal) might be a good idea. But your pdoc is the one to ask for sure.