@Etotheipiplus1equals0, I'm so sorry, and I really feel for you. (I lost my DP very suddenly too, 5.5 months ago, also a cardiac event, and I remember only too well the awful shock and despair).
Take every bit of support and help you can, and talk to as many of the people who also loved him as you can. I found talking to my DPs mother and sister especially comforting. And cry - don't ever be afraid of embarrassing people with your grief.
If you have someone close, friend or family, who can come and stay for a few days so that you don't have to worry about the day-to-day practicalities, that will help. I got some really valuable support from the bereavement board on here, too.
And you couldn't have done any more. The hospital told me that even if they'd saved my DP, his heart was damaged that he would have been really quite disabled, and he would have hated that. I found that somewhat comforting.
He sounds a lovely man, and I'm truly sorry for your loss.