I know that's how you say it...but there is no such thing as "could of," "would of," or "should of."
It's "could have" (or could've), "should have" (should've), or "would have" (would've).
Pick it apart. Take the "could," "would," or "should" out.
Do you way, "I of eaten my sandwich?"
Of course not. You can say, "I have eaten my sandwich."
Therefore, you must say, "I could have eaten my sandwich, except that I had to answer the phone." You must say, "I should have eaten my breakfast; now I'm hungry." You must say, "I would have eaten my sandwich, but the chocolate cake looked better."
Oh, and for the record, it's means "it is." Only and forever. If you write, "The cat chased it's tail," you really wrote "The cat chased it is tail." Its is possessive. There is no such thing as its'.