This is not a new answer to this question but my thoughts on this are unchanged.
First figure out exactly what you are looking for and work on vocalizing it. I suggest writing your thoughts on paper so you can read them and see if it makes sense. Be specific if this is a want or a need. If you don't get a want you probably won't die or end up doing something that will jeopardize your relationship. If you deny a need however, you may end up unhealthy and not fulfilled in life which is a form of death in my opinion.
New addition to my old comment
***Identify what you feel you will get out of having both of them in your life.
***Look for positives that your fiance will get out of it other than seeing you get what you want and being happy. This is the hardest point in my humble opinion.
Clarity, clarity, clarity. Don't leave your partner guessing or confused. Be 100 percent honest and don't try to put a good spin on things for the sake of your partner. Get it out very precisely so you can begin the work and move away from defining what you are trying to say.
Hopefully this helps a little
Playing the Game of Life with Monopoly rules.
Monogamy might just be in my genes
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