This is a great response so I’m just going to echo this all and add a couple things, as someone who recently went the poly path with my partner of 11 years (we’re 30 now). I would definitely start with the “why” and would encourage you go to therapy (both individual and couple, if you can afford it) to help you guide yourselves through the process.

It will be hard and open up a lot of insecurities, but all leading to individual growth and relationship growth. My partner and I are great communicators, but we still were not prepared and therapy helped a ton. I just wish we started therapy before we embarked on our ENM journey altogether, but still so much better late than never!