Hey there! Thank you for being so open with your personal life. It sounds like you really do love your fiancé and there is nothing you find disheartening about your relationship with her. Perhaps then you need to do some internal work with yourself and ask some challenging questions. Look deeper into how you formed relationships with people from early childhood to your manhood. Was there neglect? Abondonment? Rejection? If we experienced these emotions and don’t get a hold of them, they tend to affect how we bond with others later in life. Maybe you are scared of fully committing every part of you with your fiancé? Who knows. But if you look deeply into it, and if your love for your fiancé and her happiness means everything to you, I believe you will get to the bottom of the root of this cheating.
All the best!