Why do I keep getting seriously involved with guys with personality disorders?

It just hit me after yet another devastating and confusing breakup with someone I was in a serious relationship with (he broke up with me in a total irrational upset), that looking back on the few serious relationships I've had in my life (other times I'm mostly solo/independent), each guy has had some form of mental disorder that was obvious yet untreated. My dad's bipolar, and I wonder if I subconciously pick guys who are mentally unstable? Please help me figure this out. Could it just be a coincidence, a streak of bad luck? What can I do to find a healthy relationship instead?