what's missing?

my latest review by my boss was about how he saw potential in me, but somehow i've not met his expectations..
he "blames" it on my inconfidence
since starting on this job, i feel like i've been told that i lack confidence a billion times. is this true?
i never thought of myself as someone who did not have enough confidence.
(mabbe i just wasnt thinking ;P)
but these days i'm beginning to believe that this is true.
i keep thinking that i'm not good at what i do, not good enough.
how do i gain confidence?
how do i just be confident?
whose recognition do i need?
am i really inconfident?
is there another reason for my lack of performance?
what do i need to do?