It might just be me but I think the longer you wait to bring the subject up again, the more likely she will feel your not acting on your desires has you in a resentful relationship with her. And to try to find comfort in you exploring a new relationship with someone who only needs to smile to brighten your day while you're on the resentful tip with your monogamous wife might be too scary a risk to her. It could make her feel like you're setting her up to be cast aside for something lighter and less difficult.
This has come up for lots of people I'm betting. If the new person and you are all caught up in the 'Wow you're like my soul mate' feeling, it can make your other partner who went though the good and the bad with you feel like all the time and work they put in is being devalued for how much easier someone new got into your heart and mind. My husband immediately felt this when he learned my BF had expressed love for me. "He has faced absolutely nothing in the way of a struggle with you and has never had to prove anything to you; of course he thinks he loves you!" was his reaction. And we are both poly; me coming to it because I knew he idealized it. If that is how he reacted, I can only imagine what it would seem like to a mono partner of someone wanting to become poly.
Do you have a clear path in your mind for how to prevent your wife (and you!) from feeling that especially when she currently and potentially will always struggle to identify with why you want/need it so much?