"I have this friend who I've known longer than my fiance and I've been in love with him for almost 2 years, but I also truly love my fiance....I keep worrying that my friendship with my friend is going to negatively affect my relationship with my husband-to-be if I continue having these feelings for him."
You have to decide if this "Love" for the person you have known forever is that of a deep friendship or a romantic love - when you claim a strong physical attraction it is more likely to be the second, whether or not you act on it. If its the second, you have no right to become engaged to another EVEN you let them know this up front.
"How do I let my friend go, temporarily, while explaining to him WHY I am letting him go?" Temporary won't do it, not with the way you feel. Either let him go completely or end the engagement. Coming back to that at a later point is even more dishonest.
You are cheating your fiance out of a relationship that has you wholeheartedly in it. Don't do it to them, or yourself. Work out your feelings BEFORE continuing this engagement and certainly well before actually tying the knot.