Continuity is a big problem for me in my 21 book mystery series - I have an Excel file for the characters including the minor ones who may or may not need to be re-used. I can usually keep track of the main recurring characters. One or two of my readers have alarmed me by announcing they were going to re-read the whole series!
I am never sure about voice, and if I re-read one of the previous novels in the series I am usually struck by how much worse the one I'm actually working on is. My natural vouce is quite light-hearted which is ok for this particular series, but it's hard to tone it down enough for more serious or historical writing without being accused of being too earnest.
Considering how often I get emailed about someone rereading the series for the umpteenth time, I'm right to worry excessively about continuity for my series. I live in fear of making a huge mistake that can't be undone without erasing a book from existence. Almost did it with one, where I set up the whole main plot based on something that I remembered incorrectly. I tend to retcon a lot as I go in a book. :D
For that series in particular, I do have an excel spreadsheet set up for my timeline. The whole thing overlaps book by book too extensively to keep it all in my head. The universe is huge, at this point. I wish I could write little standalones and never have to worry about this kind of thing, but that's just not me.
Maybe it's a common thing for writers--to think the current book is the worst. I am there with you!
Objectively, I can look at my last published book in that series from more than a year ago, and I'm pretty sure it was a really good book. I thought it wasn't going to be, when I was writing it. I do remember that. It also didn't feel like one of my books when I was writing it, but I don't think I can be objective after all, because I just don't know.
Truthfully, I've been dealing with some mental stuff the last couple of years (peri-menopause is messing with my head more than my body, I think) and I really probably can't be objective at all on any of this. I do know my writing and my motivation to write (not money, the internal stuff is all that's ever worked for me as a motivator) has taken a huge hit.
And I keep thinking every time I open a finished book of mine that my new stuff just doesn't sound the same and is worse off for it.
I swear I even feel sometimes like I've forgotten how to paragraph properly.
Which is all enough of a warning sign that I've tightened up in my writing.
I'm sure I'm probably trying too hard to sound like *me*, which is totally screwing everything up, because a person sounds how they sound and the minute they start *thinking* about it, they sound different.
LOL. What a brain dump. Sorry guys.