Tuvix was three poeple.
Tuvok, Neelix and Tuvix.
Janeway killed three people to make two people.
A couple weeks later, Kes dumps Neelix under the thrall of an alien Warlord. Off screen, after the credits (I hear Ethan supposed at a convention once.), she must have confirmed Neelix that the Alien symbiont controlling her, didn't give a shit so well about his feelings as to be more honest about her own waning feelings for Neelix, which she cared too much for him to be so honest as to mention aloud that the site of him was beginning to make her sick.
What if Tuvix occurred after Warlord?
No Kes in Neelix's corner, browbeating Janeway.
Kes must have loved him less after the Tuvix incident, because of the Tuvix incident.
Is it because Neelix was less Neelix than he used to be?
NUlix was still too much Tuvix and Tuvok that he was not the same magnificent man that she fell in love with. that this half Neelix they were trying to pass off as the real McCoy did not cut the mustard.
Which could have just been her imagination.
A months separation led into Kes creating an idealized version of Neelix in her mind, An UBer Neelix (Imagine a hedgehog that was almost supersonic.), who she prayed would return from that Quantum entangled mess and make her toes curl and... An impossibly cutting figure which Neelix could not possibly compete with.
Neelix liked being Tuvix and resented Kes for murdering him, and that resentment made him treat her differently and it pissed her off to the point that the Warlord expressed the little girls distress for her, and they're broken up.
kes couldn't forgive Neelix for wanting to be Tuvix and for abandoning her. You know like how idiots blame rape victims for being raped and think of them as damaged or unclean (i'm looking at you "Everyone in India".). Westly clearly said that death doesn't stop true love, it just slows it down a little, and the hedgehog wasn't even dead but he still preferred to snuggle into Tuvoks DNA rather than her warm embrace,
Mingling with Tuvok like that, was in a sense cheating on her. And I don';t mean that Neelix was cheating on Kes. I mean that Tuvok was cheating on T'Pel, and more importantly that Tuvok wasn't cheating on T'Pel with her. She clearly wanted the Vulcan bad but had respect for his nuptials that she didn't tap that, but have you ever been in a threesome and somehow found that you're just playing with your self as the rest of the menage a trois rolls over towards the other side of the bed?
She loved Neelix enough to kill for him, but not enough to let him be happy.