Everyone is gifted in different ways. You may not have discovered
what your natural gifts/talents are (and weaknesses too). Maybe
your teacher/mentor can help you. Take Britney Spears for instance.
She may be last academically in her class but she's tops in
singing.
When you're young, often you don't know what you really like to
do/become.
So the onset is primarily trial and error. Expose yourself to do
different things so you/your mentor can gauge what you really like
and are good at. For example, some really like to sing, some
swimming, some soccer, some modelling, some like to taste and cook,
some like fashion and/or jewelry design, some like to
draw/paint/craft, some like to study scientifically, some like to
think business, some like to interact and deal with people
(teaching, PR), etc.
Once you've tried different things and can identify say 3 areas
that caught your passion and interest, learn them well. It'll build
your self-confidence.
Not doing well in one thing doesn't mean you should lose
self-confidence completely. Take time to discover.
As you move from Os to anywhere (poly, pre-U or U eventually), the
one thing they're (school, employer) going to look for in you is
what's your passion in. Is it just good grades and nothing else
outside it or good-enough grades with other fond interests (sports,
art, music, business, etc).
No one is a failure just because he/she failed to do something
well. You may have some hidden talent waiting to be discovered and
developed that far surpasses everyone else in your class, if not
the whole of Singapore.