I’m so sorry you are going through all of this. I think anyone would be feeling pretty overwhelmed.
An infection in the coffin joint is a serious matter. Based on the story you relate and the fact that she is still unsound, I am thinking it is not likely she will be a fully sound riding horse in the future. I think it is reasonable to be thinking seriously about her future. Since you are feeling a lot of stress right now, it’s not a great moment to make a big decision, but I agree with others that switching to cheap but safe pasture board is probably a good move. Give the filly–and yourself–a little bit more time to sort this decision out. Get the next vet check, maybe get a second opinion from another vet. Many people are uncomfortable talking about euthanasia and I’m going to mention it here because I think it is one of the options that might be on the table here. It costs a lot of money to keep a pasture pet for the rest of its life, it’s well over six figures. Not everyone is in a position to do that. If you are struggling to pay other bills in your life, it is a very reasonable (although heart breaking) decision.
Re your other medical bills, I’m so sorry. I guess you could ask the barn owner for help, but I think that could backfire and cause tension and awkwardness if what you express somehow comes across like you think they are responsible or owe you something for the accident. I think the better thing to do would be to work out some kind of a payment plan with the hospital or whoever is billing you and just make the smallest payments possible until your finances are more solid. Do not pay the medical bills with a credit card or another type of loan, it’s better to work with the hospital or clinic.