How to harmonize recorded tracks

Getting this to work properly is just about impossible using currently available technology, I'm afraid. Even just drinking a cup of coffee can make your voice sound different, especially if it has milk in it...

The problem here (as you've discovered) is that your ears are very sensitive to changes in the human voice, and even the slightest thing (including the weather) can make it sound 'different'. Any app that changed your voice, even slightly, is going to stand out, and actually make things worse, not better. Tiredness make a big difference too - no app will fix that!

There are a couple of generally accepted solutions, and they have one thing in common, and that's that they involve breaks, eventually. The first is that if you are doing chapters, do a whole chapter in one go - no breaks, nothing. Then have a small break before the next one in the finished production. If you need to go back over bits of it, then do this too, all in the same sitting. The second thing is that if you can't do this for whatever reason, then use something else (music, atmos, whatever) as a link between blocks of recording - this avoids the immediate changes, and they are the noticeable ones. This has an additional benefit too, as it's breaks in speaking that allow listeners to assimilate what they've been told.

There are plenty of things you can do to minimise the effects, but probably the most important one is to realise that it's actually an issue - so make sure that mic positions, etc are as little changed as possible.