I am a little surprised that you are paying the shortfall separately, with it now being unsecured it should have gone into your DMP so that no preferential treatment was being given to any one creditor.
Notwithstanding that though, I agree that an IVA may be an option you wish to consider, so visting
www.iva.com having a look around and taking time to speak to 2 or 3 providers before you make your next move would be a good place to start. By the way, if you do look at an IVA, or even bankruptcy, as a solution here, then as you are joint and severally liable for the mortgage shortfall with your ex wife, you need to bear in mind that they will then chase your ex for the full remaining balance on that debt. All that said, of course, you certainly have nothing to lose by speaking to a professional re this matter, you may well be pleasantly surprised.
Best of luck and do let us know how you get on.
Regards.