I have not experienced this in long term relationships, but I have had a relationship crash and burn because of anxiety.

Just don't let your worries change who you are. If you love him/her, all you can do is love him/her and act in the ways love compels you to act. It is good that you want to be good to your fiance but you cannot get caught up thinking you are not good enough or worrying about being perfect. You can only be yourself, a glorious imperfect loving person. That's also who you're marrying, by the way