Been there, done that, except with a wife not a fiancee
First. Discuss this with your spiritual father (I am not arguing over semantics, I considered my priest my spiritual father prior to being officially a catechumen or returnee in my case.) Get his advice on the matter
Two. My wife is the same as your fiancee. I do not know if it is a woman thing or just some people are different. I am definitely like you. Truth and doctrine are paramount. Feelings are fickle.
Three. My return to the Orthodox Church created upheaval in my relationship. My wife had no prior exposure to Orthodoxy and to her it appeared strange. But I was determined to stay and returned a chrismated Orthodox for three years, thanks be to God.
I attended her church for a while (a non-denominational evangelical mega church wannabe) but could not suffer it. I kept my mouth shut through all the hoopla that passes for worship and through the out of context sermon that I had to endure (yawn). But I persevered. She attended Orthodox services, occasionally, but she has no interest. To her it is not the true church, it is just another denomination.
Anyway, where am I going. Straighten this out before you tie the knot. Will she agree to be married Orthodox? Will she agree to raising any future children Orthodox? At minimum will she be supportive of your Orthodoxy while remaining separate.
I have a loving wife who is still not Orthodox. The people at the OC that I attend have been marvellous in reaching out to her just in love, no preaching. She truly loves them and often attends OC social functions.
Patience brother. Pray. Seek Counsel. I will pray for you