Risperidone is sn antipsychotic medication used for some bipolar disorders and schizophrenia, so I am assuming that when you say BPD, you are talking about bipolar and not borderline. If it is borderline what was your psych thinking!
The effects of risperidone for weight gain is fairly well known and the weight gain is often related to changes inblood sugar and the processing of carbs. The effects should lessen over time with natural weight loss occurring.
Holding the weight after ceasing the meds can happen, but with a normal healthy eating program it should shift, possibly slowly, but it will go.