As someone whose eyes have always been larger than my ability or time available, I have had to learn the value of limiting the amount of material I work on at a time. I also discovered through my cognitive psychological research that there are some pretty strong scientific reasons for limiting the amount of material one studies at a time.
There are no hard rules as to how much material can be handled but there are a number of principles which impact this. To start with there is the size of our short term memory (STM), when the STM is full (usually 5 – 7 “chunks” of material it will begin to dump information and unless it is immediately rehearsed it will be lost. The amount of material, or number of notes which can be put into a chunk (the technical term BTW) is influenced by our ability to recognize patterns and group notes by these patterns. Theoretically you could hundreds of notes into a chunk. I would imagine the stories of prodigies such as Mozart reflect his ability to quickly recognize note patterns and identify and store them quickly and efficiently.
I have toyed with writing a book about this whole process, but seem to need to limit my material at this time. This is a long and complex subject, but the key point would be to try working on shorter segments at a time and see if your learning rate doesn’t improve.