What do you do when you want to help someone but can't handle it? :/ I feel bad. I want to help my…

What do you do when you want to help someone but can't handle it? :/ I feel bad. I want to help my friend. I know shes in a bind and its hard to find childcare... but her 2 yr old is getting more and more testy and not behaving for me and just being harder to handle and idk. Its stressful. I told her I'd watch her til June 16th but idk if I can keep watching her that long. I need to rest and work on the house... but I don't wanna let her down either... :( idk what to do. it was easier to justify the stress when I was getting paid but her baby daddy stopped paying child support like they'd agreed so now she can't afford to pay me....