2B1again, I have seen other threads discussing the issue of memory gaps. It sounds like a common dilemma among WS. We know that in order to help our BSs heal and regain trust, we must be completely transparent. However, for some of us, the actions of the Affair (lying, deception, selfishness, callousness, disregard for established morals, sex with someone else, emotional withdrawal...) were so far outside what we at one time thought ourselves capable of that the whole time period really is in a Fog.
My Affair lasted 58 days. DDay was August 2012. I know that even now my BH has a difficult time accepting that I really don't remember the A as clearly as he expects I would remember such a major event (series of events). We struggled with this one quite a bit in the first months after DDay. Fact is, the whole summer kinda blurred together. I gave him the most accurate timeline I could and as many details as I have. I still do have a journal, like badchoice suggests, although now I write less frequently than I did. I have often imagined that we were in a movie and the director inserts a flashback and it all becomes crystal clear, but such is, of course, not the case.
Do your best to avoid TT, especially if you are actively holding on to any significant detail. (Significant to your BS, not to you.) I found it easier to work on the timeline when I had my calendar/ planner in front of me and was relaxed (as possible under the circumstances). Let your BS know that you will continue to think on it and work to remember events. When you do remember something that should have been included in the timeline, be upfront and reassuring that this info you are about to add was not intentionally left out. No matter what we WSs do now, we cut our BSs deeper than we can possibly imagine. The last thing we truly remorseful WSs want to do is pour lemon juice in the wound we ourselves caused.
Best of luck.