This has always been just my opinion but I believe we store every memory we have somewhere and it is not brain cells. (possibly the Hameroff microtubules theories but who really knows) But where ever they are I believe it is the recall system that needs the exercise. How many times I have been caught mid sentence losing my thought (that feels like it was circling around my head) and then remembered once I latched onto it. The memory was not lost. The recall mechanism eventually can remember most things if triggered correctly. I will say though that some forms of forgetting are a blessing and that the forgetting mechanism serves a purpose too.
Well, Freud would agree with you there, he seemed to think that the only reason we don't remember something is because we unconsciously repress uncomfortable memories or thoughts and memories associated with uncomfortable elements floating around in our psyche. Through psychotherapy or other means such as hypnosis, repressed memories can be brought back into consciousness. I don't really buy into that theory, though, it seems to have too much faith in the human brain. I think it is more likely that we sometimes forget things because we don't pay enough attention, or just because our memory itself is faulty or can get damaged (An extreme example of what I mean by "damaged" would be the people that get amnesia as a result of brain injury). That doesn't mean I disregard repression as a whole though.
On the topic of ketamine, I used to do it semi-regularly on 2017 (I think biweekly at its peak), but then more and more sparingly, and last year I stopped altogether. I had another experience with it a few weeks ago, and it was one of the most intense K-Holes I've ever experienced. Insanely visual, emotionally intense, and full of OBEs. IM'd 90 mg and smoked a little bit of weed. I remember when it started, I saw myself climbing a Mayan pyramid, and when I reached the top someone kicked me and I fell down infinitely, and as I fell I saw the mayan pyramid grew larger and larger and mutate into different patterns and shapes. then the typical Ketamine madness ensued at full force, sensations of being stretched and pushed, falling down through dark tunnels, experiencing myself as a particle, music becoming a physical place, etc. The last crazy bit I remember is that at one point I saw myself in a hospital bed, surrounded by doctors wearing clinical face-mask and big, round, opaque glasses, while they performed open surgery on me. It was insane but I wasn't frightened or anxious, I just went with it, lol. Right after that I came out of the hole.