My ex husband has stopped paying towards our mortgage?

Help I need some advice! My husband and I have separated and been living apart for sometime now. He had an affair and has since moved in with his new girlfriend. We still see each other as we have a little girl together but we are getting a divorce and trying to move forward with our own lives. When my ex left I was forced to put our home up for sale (which is not all bad as I wont to move away to start a fresh). Until the house is sold my ex promised to continue paying for his half of the mortgage. However he has since moved into a new place with his new partner and said he cant afford to pay his half of the mortgage anymore
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