Why are you getting married? What's the benefit to you? Are all the kids yours together?
I'm very sorry about your health problems, but in spite of the fact that he took care of you, getting married complicates your whole situation immeasurably.
You wouldn't be the first person to call off a wedding, and it seems to me that the embarrassment, discomfort, and expense involved with that PALE in comparison the the difficulty of divorcing.