Only two ways : don't mean to discourage you, but the sooner you
come to terms with the truth of the situation, the sooner you can
reformulate your plans, to either move on to another profession (at
least for the time being), or replan your attack to still persevere
to get into Medical School.
Either :
You're a straight As student from a top JC, then you've got 20%
chance of entering NUS / NTU Medical School (since approx. 1000+
students with straight As compete for the 200+ places available,
and yet every year PAP imports thousands of foreign doctors because
PAP says there are not enough doctors, well that's coz there aren't
enuff local Med Sch places available for deserving
Singaporeans!).
Or :
Your family is helluva rich, and you can afford loads of private
tuition to get straight As for private A levels (in spite of not
ever attending JC), *and* (even more importantly) you can afford to
attend a recognized Medical School in the USA, UK or Australia
(which will cost you over a million dollars).