Self-esteem. The measure of your own self-worth, your ability to love and like yourself, to believe good things about yourself.
Yeah, I don't have it. Seriously, my self-esteem is so broken I flinch when complimented, and if anyone says anything to really pump me up (like my poor unfortunate boss tries to do) I completely recoil and can't process it. It physically hurts to accept a compliment.
Of course, I mask this well by having great self-confidence, but there's a huge difference between KNOWING you can do something and BELIEVING you are good enough to do something. Unfortunately, it's a case of having to fake it 'til you make it, but as anyone with an anxiety-based disorder will tell you, the task is compounded by a constant fear and anxiety - how the hell are you supposed to believe in yourself and value yourself when you're always scared? Of course, then you get frustrated with yourself and that doesn't exactly help matters either.
One thing I always try to hold on to are my friends and family, particularly my brilliant coworkers who I know believe in me and try not to let me fall. I'm exceptionally lucky to work in a place where I can be open about my mental illness and know that I will be supported. It's a start in rebuilding my self-esteem - I'm sure I'll get there one day.