You should look at it from their perspective. You are a smart person who is capable of the work, yet still had bad grades. You don't need to show them you are smart and capable of doing math - it is not about that. It is about whether you can do those things in a structured school environment. Stop trying to prove you can do math and start trying to prove that you can work within their system.
I don't know the school system in Singapore, but in the US, we have community colleges that accept everyone. Most people in your situation take a few classes at a community college to prove that they can succeed in that environment, and then try again with the university. If there are similar option in Singapore, I'd go that route.
I don't know the school system in Singapore, but in the US, we have community colleges that accept everyone. Most people in your situation take a few classes at a community college to prove that they can succeed in that environment, and then try again with the university. If there are similar option in Singapore, I'd go that route.