The first thing you need to know is that it is normal to have sexual attraction to members of the same sex. You need to find out why your feelings bother you so much and resolve that problem. Once you make peace with your normal desires, the next thing you have to understand is that it isn't possible to exorcise thoughts and feelings. Part of being emotionally healthy is accepting all your feelings and knowing when and when not to act on them.
You have said that you want to marry this man, and I assume you don't wish to act on your homosexual urges. I sense that you are afraid that you can't trust yourself to behave, which may explain why you want to rid yourself of these thoughts and feelings. If you could magically extinguish them, then the temptation would be gone.
Life doesn't work like this. We need to have our wishes and urges and learn to control our actions. This takes discipline and practice. Last but not least, you are operating under a misconception that you need to say everything to your fiance. There is no need to tell him about your lesbian urges. This information upsets him and there is no benefit to him, you or the relationship to upset him like that.
Perhaps he is afraid that you will cheat on him and this would explain why he, also, wants you to rid yourself of the urges. He needs to also grasp the distinction between feelings and actions. He needs to know that you are to be trusted, lesbian feelings and all. If he is confident that you won't cheat, I am sure that he will be more at peace with your feelings.
If he can reach this level of emotional sophistication in which he separates feeling and actions and knows that you can do the same, then you may be able to share your urges and fantasies with him (on the condition that sharing your fantasies enhances your relationship, ads. spice, etc.) If, he can't reach this level of growth, then you are going to need to keep your urges to yourself