I only see one solution for you. The best thing to do is to sell your 3 room flat now. The market is hot, u can get a good price for it. I don't think you need to repay anything to the CPF or HDB if you say your flat is paid for, meaning clear title. Therefore, u can pocket the sales proceed and put it into a bond or somethng similar. At this point in your life, u are after capital preservation, not asset appreciation. Don't put into stocks or similar riskier instruments. Than you have several options if you do this.
1) Rent a room, and retire.
2) Rent a room, and continue to work until you cannot physically do it anymore.
3) Retire straight away and enjoy your life.
If you pursue option 3, I suggest that u move to JB. The cost of living there is much lower and you can stretch your flat money easily until age 65, and than receive your annuity after that. Also, I caution you that the annuity may be of an insufficient amount to support u if you were to remain in singapore. That brings u back to the question of how to stretch your flat money, and again the solution is to move to a low cost country like Malaysia or even Thailand.
The other consideration is that people with your afflcition will only worsen with age. By 2013, the MOH will allow Medishield to be used in More malaysian hospitals. If it runs out and you have to pay for your medical treatment with your own money, than its also cheaper in malaysia. But be aware that your future medical expenses will be high.
So, I am saying that all roads lead out of this country at this point and it starts with the sale of your flat.