Awww, Barbara. Take this from an older gal, you're never really "prepared" for big changes, and sometimes they happen and you have to take it one day at a time.
Just take a moment and breathe - focus on *today's* troubles and *today's* tasks. Don't worry about all these things that overwhelm you. Yes, prepare for the future, but don't let the future swallow you with anxiety. You can do this! A lot of people have been through very similar circumstances and got out the other side ok!
About your fiance, may I suggest a little premarital counselling? This will help you both iron out whatever issues - past, present, and future. It'll also help you both realize what kind of expectations you have in the relationship, and what kind of compromises you both can make if required. You might have to decide to put up with a bit of a mess, and he might have to decide to clean up after himself and do certain minimal household tasks. Marriage always requires adjustment, communication, compromise, a bit of toleration, and selflessness (*love*, *trust*, and *loyalty*). Life is no longer just about you or just about him (for his part). You both give up 100%. But it's worth it, esp. if you both love each other. I can guarantee that in your relationship, you both will find things to be annoyed about with one another, but that's just part of human relationships. You learn to deal with it and move on. It always takes work.
Good luck! (((hug)))
And don't worry. :) Remember. Focus on *today*. You'll be ok!