I think if you’re not going to admit to cheating, to spare the person the pain, then you have to be absolutely committed to breaking things off permanently. You cannot keep the cheating under the rug and then get talked into trying again.
The way to initiate this conversation is to set aside time, without much notice, probably on a weekend, to talk. And then be empathetic but clear when you communicate that although you love them and they are an awesome person, something is missing for you, and you’ve done soul searching and realized that you can’t move forward with a permanent commitment, and that they deserve better. Something like that. Tell them what’s missing for you, if you can articulate it.