I also think about what it would be like to walk out. As you know I'm even earlier in the process than WF and I still haven't fully forgiven him. I worry that I'll never really get over the fact that he had this affair and told another woman he loved her. That still f-ing hurts and I'm not 100% sure I'll ever be okay with it. When he reverts to pre-BD habits it makes me insane and scared that I went through all of this to get back to our same mediocre pre-BD M. And then I get angry that it even OCCURS to me that D is an option, since it never had been before and he's the one that put it on the table in the first place.
I also think you should give yourself and your R a bit of a break for the feelings you're having-- I think it is incredibly common for so many right now after a year and a half of this pandemic. Don't make the mistake of blaming your current malaise on your W or your R.
What I have gone to when I have these feelings is back to DnJ (wow, I wish he would post here on Piecing one day!) What are my values and my most strongly held beliefs? What do I want out of this life? Who do I want to be? And recognizing that the feelings I'm having are fleeting and I can let them pass by. So often that immediate thought of walking is emotionally driven and passes within minutes. Focus on the good between you two and all of the reasons you are still married.
The other thing I do is go back to the basics of what do I control? Just me. Recognizing you're feeling this malaise is an important step. What can you do about it for yourself? Are there self-care opportunities you should pursue? Something new? Also, I'd question why these bad habits bother you so much. Can you disconnect your own emotional reaction from her bad habit? In the big picture, who cares? Unless it is unsafe or somehow a really serious negative for you, I'd try to not let it bother you. I was able to do this actually in the middle of my whole sitch, where I forgave my H for all the resentments I'd held for many years and stopped seeing him leaving his lunch plates around, for instance, as an infuriating character flaw and instead either picked up the dish and put it in the dishwasher (took me 30 seconds) or left it and didn't worry about it. It removed so much negativity from my life. Maybe this would help you too? Basically a level of detachment.
Finally, I see what LH is saying about the need for an R to evolve, but I don't necessarily agree that the only way for that to happen is the threat of it ending. Make it a purposeful thing to try something new together-- I remember reading a lot way back when I thought my M could still be saved by trying harder about the way that new experiences affect your brain, and that doing something new together is bonding. Are there things you can do as a couple to break out of your rut and see each other anew?
I know Gottman talks about having regular "state of the state" relationship convos-- do you guys do that? It gives an opportunity to air little things before they turn into resentment. And as WF said, you guys are facing an enormous change in your lives-- have you two sat down to talk intentionally about what you both want for the future?
Anyway, hope some of this helps.
M
Me (46) H (42) M:14 T:18, D9 & D11 4/19 - 12/19: series of escalating BDs 9/20 - present: R and piecing