I’m so sorry to hear you lost your fiancé in such sudden and shocking circumstances, and that there wasn’t a post mortem to give you some answers about his death. I’m glad that you’ve found this site, and I hope it helps a little to have somewhere to share your feelings and talk to others who understand.
There are many others here who have lost a partner and will understand some of what you are going through. For example, we have a new user here called @Lady101 who posted yesterday about losing her long-term boyfriend very suddenly while he was out on a fishing trip: Loss of my boyfriend
Please feel free to reply to that or any conversation if you see someone you would like to talk to.