Irregardless to how R is going, you have to process this all. You have three things going on at once, you have the R in the M, you have your own healing/recovery, and you have your H, working on his issues that were behind his infidelity.
You are only 4 months from D-day, that is just a blink of the eye.
You are NOT regressing, you are just following your own path through this journey.
It is called the rollercoaster because you will have high times where you "feel" like you have come out the other side and that it might all be behind you, but then the dip comes and it can be long (but not always) and the cycle repeats and repeats until you are done (and truly I do not believe it is every fully DONE, but that is just my perspective and how I think, I am sure others as far out as I or further do experience DONENESS, so do not loose heart because of my perspective).
You have so many things to process. As you process things they will lead you to other things (kind of like a really morbid scavanger hunt), and on and on it goes, the processing brings understanding, healing, acceptance, but each thing has to be dealt with and it takes time. There can be lulls in the processing, some people take purposed breaks in the processing and choose to not go there for a while to be able to function in their daily lives, but it is still sitting there waiting to be dealt with (and I think some breaks are good, we all need time to just breathe and feel "normal")
Aside from processing, triggers will also come and those can really shake things up as you have found.
Dates can affect some of us as well.
Also, the better an R goes, the better the WS is at healing, and dealing with their "why", the safer a BS feels which also allows some of us to be able to process more because we are no longer in "WS focus mode" but now have the freedom to focus on us and our trama/wounds.