Hi,
first of all, don't be depressed. I was once in the exactly same
situation as you.
I would strongly recommend you to retake maths as a private
candidate and study hard. You could also enrol in ITE or other
institutions to spend this year meaningfully, as well as widen your
interests.
Remember, don't give up hope. Entering NS will be too early now,
so study while you are still fresh. No doubt this is a difficult
period for you but please do endure and go through it.
It will also be good if you can get a tutor to help you out in
math. Coping alone may be harder, and makes you less motivated.
Just take it as your last chance to progress further, instead of
lagging behind even more.
I am sure you can do it.