Sisters, help me stay firm with my ex

AsSalaamu Alaikum sisters. I'll keep this short. I'm Islamically divorced for about a year and we have three children. My ex struggles with mental health, has no job or income, and was staying with family until they asked him to leave. He went to his dad in another state, and before that he asked to spend time with the kids and if he could stay with us for a few days. I agreed (yes, I know!). A few days turned into about a week and a half. His family said they couldn't afford a plane ticket around Christmas because prices jumped, so I found a cheaper ticket and even pitched in. That pushed my account into the negative. He was gone a month, then messaged me on a Wednesday night asking to stay a few days. I didn't reply because it was late and the kids and I had gone to bed. He showed up at midnight at my door. We had just had a snowstorm and I felt bad sending him away, so I let him stay one night only. The next day I told him to ask his family to get him a hotel, but they refused. I told him he needed to leave and couldn't sleep here again, though he could visit the kids after school. He's been gone since Thursday. Tonight he asked if I could pick him up from the masjid and I said no. I'm trying to hold my boundaries but I feel guilty. He guilt-trips me and will take advantage if I give him an inch. I want to keep our interactions strictly about the children and not reopen doors to helping him financially or letting him stay. Any advice, duas, or tough love to help me stay strong would mean a lot.