Awsome, that is not awesome! Sorry to hear of your bumpy road. Hopefully you are now in good health and on your way to a brighter future..
So you went unmatched, then managed to find a spot, meaning you are now a Derm resident? If this is the case, do you think your PD would be supportive, or is this going to be a battle? Given your unique life experience, I think you have a compelling story for switching fields. If you have a good relationship with your PD, you may actually start there by opening up a dialogue and telling them how your personal experience with cancer has sparked a genuine interest in oncology. Unless they are a total DB, I think they should be somewhat sympathetic to this and may actually help you find a spot. Having your PD on your side from the outset is going to be a big burden off your shoulders. You won't feel like you're cheating on your girlfriend as you shop around and you will be able to look interviewers in the eye and tell them you come with no baggage and the full support of your PD/Chair. I think that goes a long way personally.
Under normal circumstances, I would recommend looking for a spot first and all-but securing it b4 talking to your PD, but I think your situation is a bit different..
In terms of match vs outside the match, many applicants are able to find spots outside the match. This board is a good resource for open spots (I think Henry Ford just posted an opening), but contacting programs is not a bad idea either. UCSD is opening a program and was looking for applicants a couple months back - not sure what the status is there, but I would email Dr. Mundt to find out. Maryland also had a posting, so try them too.
You could also try the match..but I'm actually not sure how that works in your situation. If you are under contract as a resident, not sure if you can re-enter the match..but on the other hand, you went unmatched initially so maybe you can? I'm sure you've probably looked into that and know the answer already..
Anyway, I think in your situation, if you have a good relationship with your PD, I would probably start there. Most PD's are human beings, but there are a few jackwagons, so you'll have to be the judge of that. Best of luck. This is a great field and there's always room for good people, so if you really want this, go for it! It's probably an uphill battle for you (unless you get lucky) but definitely worth it. I wouldn't change for anything.