I believe in a God who is omnipotent and does not allow for coincidences. In a universe ruled by this God, everything that happens to you is a message from Him; the hard part is determining what the message is saying.

I do not know if God wants you to volunteer with people with mental disorders. I do know that there is a reason why you found that particular passage at that particular time, a reason why He allows you to suffer from BPD and bipolar, and a reason why you are wondering right now if this is a message from Him.

Have you considered volunteering in the past? If so, has the idea of helping those with MI ever occurred to you before? Are you currently stable (like protector said, you don't want to be triggered by your voluntary work; however, if you feel that you are well enough, I see no reason why you shouldn't be allowed to take a risk. Your background allows you to offer a type of hope to the people you serve that other volunteers cannot provide. And why should you have limits?)

Think about it. Pray about it. And let us know what you decide.