Be like every other person quietly deciding these things while sitting next to their partner but not effectively communicating them; manufacture an unwinnable conflict. If you place him in some kind of double bind you will accomplish either a final cathartic blowup argument, or he will cave and you will no longer respect the carcass of a partner that you're left with. Either way the object is to dehumanize, distance, and conceptualize the entire situation, so much so that it's easy to rewrite history and reduce what it was that you shared to a state that's easily discardable.
But in all seriousness... This is supposed to be difficult. This is an unimagining. This will follow you and shape you. If it doesn't you are doing it wrong. It might haunt you if you're not careful. Things won't necessarily be better, they will be different. Whether they are better is up to you and the work you do on yourself. Most of this likely could have been avoided if you were both already doing this work.
Understand these factors:
- You will not escape your problems when you leave, they will follow you.
- You will not be the person you always wanted just because you left, you'll be you. You will have to build that person if you want to be it.
- You won't find the love of your life just because you've severed this connection. That is up to chance and is totally arbitrary. Be prepared to live your life as if you're living it for yourself only, because you may be.
If you can make peace with that, be brave, and do it. Ask for divorce. If you cannot, maybe keep trying. If you keep trying, do so assuming that you must be willing to make incredibly difficult changes for this to work. He needs to be ready for that too. If both parties are not willing, having faced the cold hard truth and finality of divorce, then it is most likely time. Good luck, and I hope you make the best decision possible.