To further illustrate the point that self-deception isn't easy: if believe you're shy, you can't just make yourself believe you're not shy.

Maybe you can make yourself believe that you believe that you're not shy, but I don't think you'll reap many benefits from placebo effect - you'll still get nervous when you want to speak up or go talk to a girl you don't know or whatnot. You can't argue yourself logically into self-confidence.