I can’t tell you what to do. I can tell you that using “The One” to crowbar yourself out of your marriage is a mistake. Moving from one relationship to another without taking the time to heal generally results in repeating some version of the same situation. You need to take the time to make sense of why you were drawn into such an unloving marriage and to learn new ways to be in a relationship.
If you want to leave your marriage, leave it for all the reasons you gave in your letter. Those are the reasons that matter — not the entrance of the new guy — and those are the reasons you should give your husband if you decide to separate. Neither of you should be distracted from those realities by focusing on “The One.” You should be talking about why your relationship is failing.
Frankly, I don’t believe in anyone being “The One.” I think you are drawn to this guy because he is such a contrast to the man you are with, and he offers you a way out.
If “The One” truly loves you, he will give you a year to recover from the marriage, to find your own identity outside of that relationship and to look at him clearly (without the stardust of being favorably contrasted with your husband).
I sympathize with your desire to give your daughter a positive model for adult love. I understand why you want to give her the kind of stable upbringing you had. But it’s also important to consider whether the environment you and your husband have created for her is the healthiest.
Fortunately, you and your daughter’s father do know how to be a solid parenting team. That shouldn’t change if you divorce. He should stay in her life.
You and your daughter are both young. If you do divorce, you have plenty of time to recover and to find new love. You have plenty of time to work on making the kind of family you’ve always wanted.
I wish you well,