Wow! I don't really know what to say. It's clear that your fiance is not meeting your needs because otherwise you would not feel so giddy in the presence of someone else. This other guy is meeting an emotional need that your partner is not.
I know what you mean about loving someone but the spark not being there. I'm very lonely right now in my marriage because my AH is not emotionaly available to me. However, I think that is part of the addiction and I could be wrong because I'm so very new at this myself. I don't think an addict can meet the emotional needs of any one in a relationship because he cannot meet those needs within himself. Addiction is a very selfish disease. The only priority the addict has is getting that drug and getting high, the rest of us are on the back burner.
AS for leaving your SO, I can't give you any real advice. Is your SO taking any steps toward recovery like attending meetings and working a program? That would be a deciding factor to me in whether I stay or go.
As for your "friend", I know that it feels wonderful to have someone who will engage in meaningfull conversations and will meet your needs of affection and friendship, needs that you are not getting at home. However, this man is in a relationship with someone else and you are in a very vulnerable postion at home and this situation is a recipe for disaster. Jumping into a relationship so soon would be self destruction for you. You need space to clear your head and to figure out what you want. You need to focus on you and how to take care of you independantly from another person or relationship.
Have you gone to any al-anon or Nar-anon meetings. There is an abundance of support in those meetings and most likely someone there has experianced what you are going through.
No, I don't think you are a horrible person at all. You have needs just like everyone else but instead of looking to get them met from another person, look within yourself first.