Forgetting is an automatic process, only when you become aware that you are thinking about ‘T’; distract your mind. Initially you have to do it and after some time it will be automatic. Stop comparing your fiancée with your earlier girl friends.
Happiness is subjective experience and it is by product of your satisfaction with your present life. So stop churning your past and worrying about future, try to enjoy your present and whatever you have, happiness is bound to follow.