BPD and Bipolar don't necessarily overlap but one is often mistaken for the other. BPD patients cycle at a much faster pace than bipolar and the cycling is always caused or triggered by something though you may not always realize what it is. A BPD person can go through a number of extreme emotions in the course of a day. Bipolar's cycling is much slower and is caused by a chemical imbalance and when the right meds are found the majority of the symptoms can be controlled. BPD cannot be controlled or successfully treated by meds though they are sometimes used to make off the edge. BPD is more of a learned behavior based on upbringing.