The thing that matters is not whether you fail or not,but wh

这句话主系表结构,主语是the thing,系动词是is,表语是后面的not whether you fail or not,but whether you try or not.
that matters 是定语从句,用来修饰前面的The thing,这个定语从句的先行词做了从句的主语.
not whether you fail or not和whether you try or not是并列句,同时做了这个句子的表语