My understanding is that dissociating to some degree is common with affairs, for both partners. One idea is to work on a timeline document with your BS. She can add things that you've told her but have forgotten, and you can add new things whenever you remember something. Over time you'll build up a picture together.
Speaking from experience, as your anxiety decreases, your ability to recall will probably recover. So decreasing anxiety is something you can work on. Learn more about dissociation and PTSD, like what you can do to help with dissociative amnesia. Practicing mindfulness and acceptance ("I don't need to change these emotions that I'm feeling") seem silly, but they can have a big effect over the long term. Similarly, small things like gratitude can help to shift depression. Hobbies, friendships, getting involved in a community.
It sounds like your job is also traumatic and adds to your stress. Maybe either problem was "manageable" by itself, but they add up to tip you over the edge. Have you checked whether your HR offers debriefing or counselling services for employees? Alternatively, perhaps there is a non-profit in the area that runs debriefing / depression / trauma support groups. If all else fails, could you start a debriefing practice for yourself? See if you can find a script to adapt, but basically you're thinking through the strategies you used: what worked, what didn't, what lessons you can take from this, what you might do differently in future, and how you can take care of yourself now.
For me, one of the challenges with infidelity has been simultaneously working on recovery and having patience for things to take the time they need. Focus on the things your can control: being there for your BS, working on yourself, coping with the knee jerk reactions, dealing with the trauma and depression. The memories are out of your direct control for now, and there you (both) will need patience. As you walk the road, new routes open up. Hang in there!