I'm in love with someone but it's not the person I'm marrying
Do you not love your fiance anymore?
Yes I am in love with someone else but it's not the person I'm marrying.
I hear what you're saying, but it is wrong to be deceitful towards your partner in a relationship. You should tell them what's going on, it is very dishonest to be marrying someone you don't love. Otherwise you are hurting them, yourself and the person you truly love.
It's complicated. There's a reason I can't fully tell my significant other about the feelings I have for someone else.
It is unfair to your fiance to be in love with somebody else. You need to assess your feelings and then decide if you really want to be married.