I'm thinking of being fwb with a mutual friend of an ex if the ex is fine with it
Why do you want that kind of relationship with your friend?
Please don't try to rationalize my feelings, this has nothing to do with rationality. I'm in love with this person, even when I knew it wasn't possible
I'm not trying to rationalize anything with you right now, and I appreciate you trying to see if your ex is OK with it first before you commit with the person. But you are very defensive in all of this.
You're over rationalizing love, it's a beautiful feeling I can't control it. All I can do is try my best to love and be loved.
It's nice that you are considering your ex's feelings before starting anything but  if you are in love with this person then you want a real relationship not a friends with benefits situation.