Unfortunately, a second mortgage, whether held by a bank, a private individual, or the seller of the property is just as valid as any other mortgage. There is no way to 'stop' the lender (seller in this case) from foreclosing if you have violated the terms of that mortgage. In this case, because you are past due, you have breached the agreed upon terms of the loan, so you are subject to potential foreclosure.

What you need to do is to contact the seller, and to find out from them what they will accept in terms of payments and timing of the payments to avoid the foreclosure action.

The good news is that, because it is a second mortgage, it is in second lien position. They can force a foreclosure, however once the property goes to sheriff sale the first mortgagemust be paid in full before the second lien holder is paid anything. Because of this, and the likelihood that the second lien holder will not receive any gain from foreclosing, it is unlikely that they will push the foreclosure. Either way, if they gave you a loan to help you purchase the property, and recorded a mortgage that you signed, you need to pay them just as you would any other bank or risk having the property foreclosed upon.