It's never too late to try again. In fact, if it really help you
to progress where you want to go, getting into poly, then it would
be beneficial to go for your O levels first. O levels may have much
more content that will be tested as compared to N levels. Have you
considered other strategies to study, because your current GPA may
have been a result of an inefficient strategy applied. There are
currently many books out there that could help you, books that
build memory skills, ten year series to anticipate exam questions,
etc.