if youre afraid maybe you should wait
continue having flings, affairs and Fwbs. It is the 21 first century and marriage and committments dont exist anymore sister. You dont need all that do you ??
The why is important but if you love this person and you guys work together through hard situations and great ones then know that whatever fears u have they will be by your side through it
Don't get married. Wait until you are ready.
Why are you fearful?