Hi Beth Ann,
I'm not here often, but saw your post and wanted to reply. I believe that if you have any doubts at all about the important step you're about to take, you owe it to yourself and your partner to get to the bottom of this. Maybe talking to a relationship councilor - either alone or together? Probably alone at first would be easier for you.
My son recently went through the same thing and ended up breaking off the relationship - I'm definitely not saying for you to do this! - but he now believes that it would have been a terribly unhappy marriage for him. He felt horribly guilty about it, but I thank my lucky stars every single day that he had the courage to do what ended up being best for both of them. I admit I do miss his fiance - a lot - but she's since remarried and doing extremely well.
I guess my point is that you do need to talk to someone you trust about this, and not to feel selfish, or guilty.........
Life is too short.
Best of luck to you with whatever you choose to do

.