Trust me.. I have "not talk of OW" on my list of changes to make and that's the one I'm failing miserably at so far.. I ask myself the same things you do: why can't I just let it go? what does it matter?. I also feel the same mistrust that you do.. have had the same thoughts.. what and why is he hiding. Why can't he just decide? Remember, he's also NOT deciding to be with OW right now.
Maybe moving into the apt will be just what your H needs to get a clue. I know it's got to be so hard on you. Make a plan. Don't fuss over the OW. Have patience.. You're in for the most difficult struggle of your life but you can do it!!!