Was I wrong to have with my fiance's Grandmother?

I will help you. This is a situation where you've allowed a few excuses to give you room to play out your lustful fantasies. You see lust is a crazy thing it will make a man or woman do wild things, leave their marriages and etc. all while making good excuses! I understand where you said she made you feel more like a special guy, hay that's what mature women do, these younger breed of "gimme" broads are horrible at making their men feel special, it takes a special woman to do that! Now for the reality of it all... you had with a grandmother, people have done a lot worse, for them to judge you shows their ignorant hypocritical mentality. What I suggest for you to do is to understand you're not happy with with Susan, let her go, be the man you are and learn to have better self control. I'm a God-fearing man myself, I pray to God when I have problems of this nature, wild things that they be, don't kill yourself over this, take time to heal, get better, love yourself and meet more mature, appreciative women:)