Hi Aliska, welcome! I've only been here a few months but have found 'most everyone welcoming and friendly - with lots of sound advice and solid information.
A few months back, my wife, Becky, asked me to be willing to open up our marriage so she could explore her resurgent feelings for an old college boyfriend. Full story is in my signature link below.
Here's some of what she did right:
1. Made sure that I understood that it was not about me - no matter how much it might seem that way. There was nothing wrong with me and I had not failed in any way. This was about her feelings and not a reflection on me.
2. Made sure I understood that it wasn't because she didn't love me as much as she ever had - but she believed it was possible to love more than one man at the same time without diminishing the love for either (the classic poly argument is to consider that a parent can love multiple children at the same time and still love any others than come along as well).
3. Made certain to emphasize that her relationship with Ben would be "in addition to" - and not a replacement for - our marital relationship. And that our relationship would still be "primary" (after all we were married with a child in a home - with a mortgage, etc) - and that we could negotiate what exactly that would look like. I found the phrase "in addition to" to be particularly helpful in helping me come to terms with her desire to have a second relationship.
4. She was very patient - and did not try to push it along too quickly. We could take it one step at a time, and she agreed not to have sex until I felt I could accept the idea. And although she did want to include sex in their relationship eventually as a natural progression, for her it was much more about her feelings and her emotional involvement.
5. She was willing to talk about it as much as I needed to - no matter how long it took or how often - and to answer any questions that I might have - without any apparent frustration. She understood the need for me to process the situation. And she was very patient and understanding of my frustration and emotional turmoil at her request - very validating while still upholding her belief.
6. She went out of her way to reassure me of the depth of our relationship during this time - avoiding arguments, being especially loving and affectionate as well very open and honest about here thoughts and desires.
7. She agreed that being completely honest and transparent was absolutely essential - to whatever degree that I felt I needed that to be ok.
There is probably more that I could write about with a little thought - but Becky really did do a great job in asking me to open our marriage to poly. Hopefully some of this may be helpful to you as you discuss things with your partner.
Best,
Al