「かなしみ」は、形容詞のごく一部に使われ 「かなしさ」は、状態的なものを「程度の概念を持つ名詞」に変える という違いがあります つまり、「～み」というものは形容詞のことであり、「～さ」はあるモノを名詞的に使うということでしょうか。 そして、What's the difference between A and B というのは、「AとBの違いはなんですか」と言います。 お役に立てるとうれしいです。

The following URL explains everything. I have no additional comments. http://japanese.stackexchange.com/questions/1417/what-is-the-difference-between-%E3%81%95-and-%E3%81%BF-suffixes-to-make-a-noun-out-of-an-adjec

It's very difficut. I would translate both into sadness. The usage is not different as long as I try to find now.

It's a tough question even the diff is one char. It's lucky that you would feel the nuance in 3 years of practice. Keep reading, reading and reading Japanese novels.

かなしみ would exist in human. かなしさ could be felt by human.