You seem pretty certain that the marriage will end only on your say so. Are you sure that your H isn't unhappy and thinking of ending things himself?
You seem to be already aware of the tendency people have to project unhappiness on to their marriage when the real problem is something else entirely.
Most people just trudge along until something happens that offers a glimpse of excitement. If that's another person, it's so, so easy to justify exploring those new, exciting 'alive' feelings by rewriting history and making your marriage the reason why you feel flat/unfulfilled/unhappy, etc. In doing so, you reduce your spouse almost to a bit player in your life and it becomes easy to forget that they are a person in their own right, with their own thoughts and actions and their own agency. That's why so many people are shocked at their betrayed spouse's refusal to take them back when they try to return to the marriage after infidelity has been discovered. The fact that the cheater sees their spouse only as a spouse - i.e. a person defined in relation to themselves - rather than as a person in their own right, may actually be part of the reason the marriage is having difficulties.
So I ask you again, how would you feel about your H and your marriage if you felt he would leave you?
If you feel relief, ending things is probably the right call. But if it makes you feel anything else, seriously question whether calling off your marriage would really do anything other than add a lot of stress to your life.
The thing is, you could leave and never meet anyone else. How does that make you feel?
If the idea makes you feel free and full of possibilities for other areas of your life - e.g. a new job, a new friendship circle, a great new hobby - then why can't you do those things inside your marriage? If the answer is because you H will be awkward about it, then that's definitely good enough reason to leave. But if he'd actually be supportive of you doing any of those things, what do you hope to achieve by leaving your marriage that you can't achieve by staying - save the possibility of a relationship with someone else that may or may not happen.
I'm not downplaying the importance of relationships. It is, for most people, a natural instinct to want to have a meaningful relationship with a partner. If you feel your marriage can't be described like that and won't ever be, you don't need any further justification to leave if that's what you want. You wouldn't be human if you didn't hope to find that with someone new. And while you may not ever find the relationship you want, being in a bad one can blight your life and it is better to find happiness alone.
But I urge you to be absolutely sure that you've identified the correct source of your unhappiness before you do anything, because you'll save yourself a lot of heartache and give yourself a much stronger shot of building the life you want.