Necromancy is like reaching into that same box of knives but fully lit.
*Corection: Can be ...

Its minor difference, but important.

Yes, you Wizard *can be* educated and really well aware of coruptng powers of necromancy ...
OR and that is also possible, that Wizard can simply be some dude who managed few basic cantrips, find some grimoar, and start learning from it ... and before you know it, he ding level 10 and no longer care that his weins starts to show up, his eyes seems watered, his skin is unnaturaly pale, and his muscles seems to be atrofic.

You said it yourself after all ... Wizard "is able" to study, but nothig more.

but not whatever bizarre aberration they're experiencing now.
To recognize this you would need to metagame ...
All they know is that it's not working the way it should and they have some unknown amount of time longer than they normally would.
Yup ...
Educated person can recognize that something is different within meere few hours ... since he is still fully aware of himself, any more time just adds value to this observation.
The question that person must ask then would be: Is that good, or bad sign?
And that is totally us to your character personality ...
- they can freak out anyway, well basicaly for any reason including "simply bcs!"

... just as Shadowheart do.
- or they can be completely cool, simply presume something went wrong and they are perfectly safe ... just as Wyll do.
- and of course, they can do infinite nuances between those two extreme states.

So knoweldge of regular ceremorphosis doesn't really help that much.
Oh please ...
Have you ever seen any educational video about drugs? Dont you know how those people keep repeating the same phrase?
Its: "Im not addicted, i can stop whenever i want" ... and we all know the truth, some of them know it aswell, they just cant force themselves to admit it, bcs truth is too scary.
Same situation is here ...
Yes, some could teoreticaly find out and admit that they have problem ... but for others, the truth is too scary ... so they will rather pretend that nothing bad is happening, and they are fully in control ... until it will be too late.

Same goes on our streets ... speed limits exists for reason, and yet from statistic around 70% of drivers ignore it ... why? Simple, they all believe that they are so good drivers, so nothing bad can happen to them.
In therms of netherese ceremorphosis, your own Ego is your worse enemy.

I'll admit that I can't exactly remember when the warning comes up either.
So ... lets put this argument to ice, so it dont spoil?

But you don't actually know what you're losing.
Exactly my point my dear uncolored spirit!

Even one step futher ... we dont even know if we are actualy "losing something" our character actualy had ... bare with me:
As far as i know, Narator only describe our feeling ... so we feel like we loose something that we can never get back ...
That may indeed be part of our self ... or something simmilar, that we loose, bcs we are changing.
OR!
And i think i dont need to say that i find this option more probable, but i just did anyway. 
It can be something spiritual ... or simply collorfull description of remorse ... we know that power is "unclean" ... and yet we used it, we were "pure" before and we lost that purity, forever, with no way to get it back ... like remaining scar on our soul that will never trully heal.
Thats why used virginity as an example ...
In some cultures even today loosing it before mariage is concidered quite bad ... other groups of people simply dont give a shit ... and there are even people who seeing having it as something bad.

And yet it can be described as "you feel like you lost something you will never get back".
