Hi Thoreau-
Seems to me that there are 2 or 3 primary issues to consider in making your decision...
1. You mention issues with your business. Will things get worse (or harder) because the "market"/demand is changing. Might be a good time to make a change simply because of future financial prospects.
2. Your aging father is having a hard time with the demands of the business. I was going to suggest (as others have) that you consider taking on a partner if that is feasible. Could take some of the pressure off both you and your dad if you really want to continue with your current business.
3. Your internal conflict. What do YOU want to do? I think you probably know what would make you happy but are reluctant to do whatever it is because you feel responsible to be the "caretaker" for both father and fiance.
My best guess is that both your father and fiance are very aware of your emotional turmoil and want you to do whatever would bring you peace of mind.
IAS