Have to postpone or cancel my wedding

I’m so sorry to hear about your fiancé’s health issues. If you have already sent out save the dates, I would send out cancellation notices to everyone who received an STD. I would keep it short and simple. Something to the effect of “due to unforeseen circumstances, we have made the difficult decision to cancel our wedding at this time. Thank you for your continued love and support.”
I wouldn’t say anything about rescheduling at a future time or anything like that. Simply because after experiencing severe health issues, you never know what may be around the corner in the future. He issues may go on longer than expected, or become more costly than anticipated. You may not be able to afford to host a wedding, or the two of you may no longer wish to after battling a health concern- so I would avoid making any promises for the future.

Hey:

I'm sorry that you're going through this I really am! I know some couples who still decided to get married but did something small. The man/woman was the person of their dreams and didn't want to miss out on the opportunity if marrying their soul mate. I'm not sure if that's something you would want to do, but because she's sick this gives you ever more reason to maybe have something intimate, closest family and friends who knows what y'all are going through.

I'm sorry to hear about this! I would send a cancellation. That relieves you of the stress of having to follow up on the promise of a future date. People will understand.

I'm so sorry to hear about her health problems. You could send out something that's between a postponement and a cancellation: "We regret to inform you that our wedding will not take place on X date. Please stay tuned for future announcements." That makes it clear that you're still engaged, but just can't plan a date at this time.
I also agree that you could hold something very small and simple now, and then plan a bigger event in the future - even on an anniversary. That might take the pressure off to have the wedding looming, but would still allow you to plan and dream about a future big party, without so much stress, waiting and wondering.
Good luck!