Even though I am an avid believer in choosing to be happy, I don't believe in pretending to be happy. I like to say affirmations can cause schizophrenia because they are so misused and misrepresented.
An affirmation is a statement of Truth. You aren't trying to change what is on the outside, you say it to help yourself to remember what is really True. When you understand it in that context, it doesn't make much sense to go around saying I feel happy when you truly feel like crap. On the other hand you can look at your circumstance and say, you know what, this stuff is crap, but I'm going to be happy anyway (at least for one moment, then another, then...).