Hi
As I think you are aware, you are on a very slippery slope and are now faced with having to make a big decision.
You need to get some debt/legal advice as to the best way forward.
As Citizens Advice are very busy at the moment and it may take awhile to get an appointment, you could phone the Community Legal Advice people if you live in England or Wales. They can advise on all the various issues that you have and point you in the right direction. See http://www.communitylegaladvice.org.uk/
If the debts are likely to exceed any equity in the property, you could look into other options. The insolvency helpline has a lot of useful information and a freephone helpline. But you should also get some legal advice, so that you are not sharing an unequal share of any debt.. http://www.insolvencyhelpline.co.uk/
It is better that you deal with the situation , rather than wait until your creditors you owe money, take action themselves, when you have no choice. One thing you should never do is hand back the property to the mortgage company. I know a few people who did this in the early 1990's and they were left with massive debts, as the mortgages companies sold the properties cheaply at auction.
Once you know a bit more, you should inform your ex partner of your intentions and that they will also need to get some advice.
Huckster