I have had to drop a few long term friends for the ways they act. You can only go so far before it becomes THEIR responsibility to change THEIR actions. A mental illness can only go so far as to excusing bad behaviour. It's up to you to decide if the benefits of the relationship outweighs the stress and anxiety it causes you. Perhaps it would be healthier to focus on the relationships that give your life consistent positive reinforcement than to play caregiver to someone, but that's up to you.