@Skropi Keep in mind I have much trouble memorizing now, while I never had this trouble before. Maybe I am just tired of memorizing, and prefer to just learn to sight read? I am not sure but this is how it is.
I've studied memory for various reasons and I can answer. Doing here to not conflate different topics.
The most important aspect of memorization is…..
…. no, not repetition (even though that's important too). The most important aspect of memorization is GOOD and LONG sleep. If you are getting less than 7h per day your memory is impaired, the less you get the more you get impaired. Anecdote: I have a friend and colleague who experimented with 2-3h sleep / night + short naps totalling less than 2h/day "to get 4h more of life" (his words). In those day he kept repeating me some (admittedly important) things over and over, every day. To my complaint "we've already discussed this yesterday, remember?" he seemed honestly unaware: his mind simply had not memorized our lengthy discussion of the previous day. So, make sure you get lots of high quality sleep, at the very least 7h, better if 8h or more.
The second most important aspect of memorization is…..
…. no, not repetition (even though that's important too). The second most important aspect of memorization is practicing recall. Now this becomes a little more technical but basically our memories are somewhat similar to a computer, with a cache and a main memory. The cache not always gets flushed to the main memory (certainly not if you don't sleep, but it may not get flushed there for other circumstances too). The main memory is "sloppy" and has a poor indexing system. Practicing recall helps BOTH the flushing and the the indexing which are the two main weak spots. What is recall? Simply said, trying hard to remember, after you have "forgotten". So if I tell you my phone number is 719-342-5678 (not my actual number) you will remember it best if you try to tell it back (even silently to yourself) after you have somewhat forgotten it. So perhaps after 5-10 minutes since you heard it. Even better if it's the next day. The surprising thing is that it doesn't matter if you get it right or wrong! As long as you try hard and have a way to correct yourself (e.g. I tell you, no, you got it wrong, my number is xxx) you memorization improves. But only when recalling it from main memory, not from cache. So recalling many times in a row is useless as a recall exercise. More on how this apply to music later.
The third most important aspect of memorization is…..
…. now you got it repetition! We all know this, at least since the ancient roman (repetita juvant)
However it's a third and cannot do miracles, especially if the first two are lacking. Now, sometimes the recall and repetitions go hand in hand. E.g.: I keep repeating you my phone number every few minutes, and your mind, subconsciously and without you noticing, tries to anticipate my talk: that's repetition and recall at the same time.
So while it is possible that your sleep has a role, I suspect your ability to read has greatly affected your practicing recall. Before, the inability to read forced you to continuously recall. Now you are not doing that anymore. The good news is that you only need the recall from "main memory" not from cache. So trying to recall once per session suffices: subsequent recalls are from cache and not helpful to memorization (even though it "feels" the other way around, silly brain!) - You can practice playing and reading AS LONG AS you do it at the subsequent and NEVER at the FIRST attempt of playing. Such first attempt should be performed with the book closed. Does not matter if you get it wrong, but you do have to try hard and correct yourself (by reading) immediately after.
It is also possible that the repetitions have decreased, e.g. if your technique has improved (this happened to me): now you can play without doing it over-and-over as before. But again, this has less impact than the recall, and it is also more time consuming to fix.
So @Skropi if you care about memorizing something, give that recall exercise a try and let me know how it goes. Better yet if you are also studying something of similar complexity at the same time, which you do not care to memorize and hence don't do the exercise, while practicing it in otherwise identical manner. So you will see the difference between the two approaches. I know from theory and practice what it is for me and for most people, but many don't know it and are shocked when they try and see the results!