From my experience and rather narrow point of view, bpd comes from periodical surges of inner feeling of connectedness there where usually is chronic disconnectedness (waves of secretion of the respective 'connection' chemicals in the brain there were the chemicals are usually not secreted or secreted with severe deficit).
So usually one feels down, unmotivated, foggy, lost, and then seemingly out of nowhere these bursts happen of immense energy and hope and belief in good, love etc. usually as a result of some more or less subtle triggers, like minor good events (these triggers can be very hard to identify at times).
The idea is, those surges of chemicals are normal, that is what should happen continuously, the manic behaviour comes simply from the fact that the person can't maturely handle those chemicals, the body doesn't have [consistent] experience with it, for it to have a balanced and productive response to it.
Give money to the chronically poor, and he will be overwhelmed and emotional and unexperienced and will throw the money away thinking he is investing. The same here.
I don't have a practical advice at all, other than see if the above feels right to you. And if it does, to just sit on it and explore the thing further.
