Bad credit is very relative. With a 620 score, you could possibly be on an FHA loan with your fiance. The difference in interest rate between a 620 and an 850 credit score on an FHA is really often no more than 0.25% to 0.50 added to the rate. Also, you could pay (usually one point or less) to get the better rate.
There are also Lenders who will go as low as a 560 score with about a 1.25% rate increase and maybe one discount point. It is an option should you want to consider it. They look beyond the score and do a "common sense" underwrite, taking your whole picture into consideration.
I know there are many, many snarky naysayers in our market today (unfortunately) who treat Borrowers with less than "perfect" scores almost with disdain, but please don't let them dictate who you are.
Your credit score is a very important factor in obtaining a mortgage, but remember that...it is really only one of several factors. Your income, time on job, down payment and reserves are just as vital to getting approved, but somehow, everything else seems to have been relegated to being almost non-existent without the "almighty" FICO score.
Please don't let yourself be stalled by what you read and hear. Get in touch with an experienced (and human) Loan Officer. Even if you can't be on the loan, you will get tips on how to improve your credit, and if you so wish, you can be added in a later refinance.
I would be happy to take a look at your situation, should you so wish. Please don't go out looking at property AT ALL, until you have a credible preapproval letter in your hand.
Good luck to you and your fiance, and here's hoping you have a great life together and create fabulous memories in whichever home you purchase....whether you are on the loan or not.