2nd Mortg. in collections

I was just recently divorced and have been working with b of a on loan mod. I asked several times if I should continue paying on the 2nd and was told not too. As long as I was in a loan mod. process I would be fine. Should have known better. I had to stop paying because the mort. was too high. I was recently informed that my 2nd mort. $19,000. was sent to collections. Apparently there was undisclosed pmi on the acct. This collection agency wears many hats, insurance co., collection agency, etc. The collection agency verified that they own the debt and b of a is out of it. They sent me a letter and I am not sure how to respond. This is all new to me.....I do owe the debt can I negotiate it down? Dont have $19000. Im current on all my other accounts. Should I call them or write a letter?? HELP!!!!