I feel like this is that whole "don't look at your art for a day/flip the image" effect but with a larger, more noticeable gap. You can only see all the flaws when you check in on old art, when at the time that you draw it you usually only see all the ways you've been improving over the last instead. It takes practice to look back and appreciate old art for what you did well, same as it takes practice to work out what's off or not quite right with current art.
