Hey Human,
Things like these are often difficult to cope with, and I understand your plight. Having to deal with such a dilemma right before marriage especially with the woman you love can be quite stressful and hurting. In times like these, the most important and quite certainly the first thing that you should do is have a proper one on one conversation with your partner regarding this problem. The lies that you are talking about, they must be confronted as it is hindering your mental health, and hence it demands a conversation which is separate from a confrontation. Asking questions and being patient with their answer is of prime importance, as not being patient in such a situation often leads to fights and that doesn’t serve the purpose at hand at all.
This being said, in situations like these, some couples also choose to go to a relationship psychologist for advice. These people know hoe to handle such situations professionally and are more qualified to give you an answer that will benefit your relationship. Right now, what’s beneficial for you will be known by you both collectively. You both are in love, and there is a marriage coming your way. Before thinking of a harsh step, you can always consider options like a relationship counselor and if that becomes a stretch for you both, a proper adult conversation about your problems as a whole will not fail you.
I hope this helps you, and I hope you find your way soon!
Next Steps
Try couple's therapy, which will help both you and your fiancé, you should also try therapy individually to give you a better understanding of what you are feeling.