It depends how lousy you mean. If you are mostly B with many As kind of lousy then there is still some hope. If not, your only chance is to enrol in university, get a fantastic GPA there and apply for graduate school. If scores still lousy, then try chinese medicine.

There are many ways to becoming a doctor but they all require money. If finance is a problem for you, then either you go local or you go to malaysia. The twinning programs in KL IMU and penang medical colleges are with recognised med sch and recognised in singapore. Quite a few ppl with lower grades go that path. The first three years are paid in RM so you save quite a bit.

If you want to study medicine, there are no cheap medical schools unfortunately. Unless you have Aust PR, then it cost only 4-6k a year but you will need to have all As.

Another way is go india or russia. They are much cheaper and grade requirements are lower. But because they are not singapore recognised, you will have to stay there and get a specialty before you can come back. This road is LONG but who knows, you might like it there.

Another option is carribean medical schools, these provide a back door to US specialty training. Not that expensive either.

If your aim is for "DR" prestige, forget it. Patients treat doctors like waiters nowadays. No longer respected. No thank you or please. They order you around and question everything with the stuff they find online.

If you aim for money, then do finance/banking/accounting. The path to a specialist is very long and during the training years, trainee pay is below market rate compared to your peers in other jobs. If you have a family, it wont be enough. i know for a fact trainee pay is less than a secondary teacher pay. By the time you start earning decent pay, you will be around mid thirties to fourties. Not really worth it.