Done. So why is there not a comma? I don't understand why. "I've seen it" is a complete sentence in itself and adding "myself" adds emphasis but it doesn't have to be there. Does that mean that "You've seen it yourself" is correct, as well?
If "myself" or "yourself" were far apart from "I" or "you" then I might use a comma. But I wouldn't use a comma in a sentence as short as these. Nor normally would I pause before "myself" or "yourself" in speech.
If I did want to indicate a pause before "myself" here, I might use a dash: I saw it - myself.