I've never commented to you before, but I know exactly the feelings you have described! I, too, have had nightmares that involve H with ow or H leaving me. It just goes to show that the fear is still lying beneath, even when we don't realize it. I have done great work on myself and know I'd be fine had we not reconciled, but the fear still lingers. I think we need to be patient with ourselves and realize that we have experienced a form of trauma. Recovery takes time and consistency, plus, like May suggested, finding healthy outlets for our stress (I'm a Pilates junkie). I'm glad your husband, like mine, tries to lovingly reassure you when you feel that fear.
Additionally, I've found that I still have a lot of anger and unforgiveness, not toward H but toward ow. I think it's because not only is she not regretful for her part in trying to destroy my marriage and family, but she actually delights in it. She suffers from some diagnosed mental problems and has tried to do things like run me off the road in my car, "trap" me with her shopping cart on a grocery store aisle and say horrible things to me when she walks past me (always with a bump into me). I know that by not letting go of my anger and by not forgiving her, I'm only hurting myself, so this is something I'm working toward daily.
I didn't mean to hijack your thread, but I'm very glad you started it, as I was thinking of starting a thread in piecing that addressed how we can go about letting go of the fear, anger and unforgiveness in our hearts. I hope someone who has been successful at this will chime in to give those of us still piecing some much-needed advice.
In your case, I bet your current anxiety has to do with time. You mentioned your H was with ow last summer, so is it coming up on the dreaded bomb dropping anniversary? Mine was a year ago April 29, so I've had some hard feelings to work through lately. It's important we don't take it out in unhealthy, unforgiving ways to our Hs and instead work through it in healthy, positive ways.
I wish you the very best! You have a lot on your plate right now, so remember to give yourself a break and be kind to yourself. That goes for you, too, May!