To Comma or Not to Comma

I struggle with commas. Yes the barely noticeable little comma.

I'll compose a sentence in an email, letter or blog post and insert a comma where it seems to be called for. Then read the sentence a day later and wonder what the hell I was thinking .... that the comma obviously ruins the flow of language and meaning of the sentence.

How can that tiny mark of punctuation one day seem so needed for proper understanding and so unnecessary and plain wrong the next? 

There ought to be an upper division college course that focuses on the use of commas.