The filly didn't make it through the harsh winter... Where do you live? Did you just let it out to fend for itself in freezing cold, or? I mean, yeah, winters can be harsh (I live in Minnesota, I know my harsh winters) but that seems strange. What was her documented cause of death? (just curious)
And I agree with the majority. It DOES sound spoiled - and it IS possible to spoil a horse unintentionally, so you may not even know you're doing it. I'm not sure how to solve the problems, but a sudden change in behavior isn't normal. It hints at something bigger - either the amount/time/frequency/method that you work with her has suddenly changed (for the worst) or she's having a medical issue. I'm sorry, I hope you can get this all sorted out.