Believe in You

Why is believing in yourself one of the hardest things to do?  Eer or should I rephrase that to "one of the hardest things for ME to do?"  I am a constant cheerleader for my family, my friends, hell complete strangers even get my vote of confidence.  However, when comes to patting my own back or having faith in myself I'm at a lose.  I've tried to over analyze why I'm so hard on myself.  Why I need constant reassurance that what I'm doing or what I did was great! - not good but great!?  The truth of the matter is I have no idea why it is so difficult for me to believe in myself.  I should be my own biggest fan but instead I often feel as if I'm on a whole other team.