Ok here are some possible ideas to help you:
1. Talk to your parents, as they might have had the same problems at your age.
2. Drop those friends if it at all feels that they are making you insecure to be around them.
3. Talk to your doctors. They will know if a person your age is actually overweight.
4. Also talk to your doctors about treatment if you have not already. (So sorry I don't know much about bipolar or BPD.)
