I think it was in Sword of Destiny or The Last Wish that she went uber-bitch on him, not sure.
"Triss on the other hand may love Geralt, but her lapdog personality made me like Yen over her as a character, in the books at least."
Triss is a lapdog because she feels helpless. She feels helpless because she knows Geralt loves Yen more than he loves her. She feels desperate. This makes sense.
Yen is a bitch because why she was an ugly hunchback when she was a little girl, and now that she's a grown woman with a nice pair of tits and a magically crafted body men want to fuck her, and she resents them for it. That same emotional damage is why she treats Geralt like shit.
Here's the thing though: Literally all sorceresses go through the same thing. All of them were born ugly (their magical talent warped their bodies, or so they say - and marriageable girls wouldn't be sent off to be made into sorceresses anyway).
Yennefer has no reason to be so much more of a bitch than everyone else, including Triss.
Now maybe it's just me, but I feel so much more inclined to like Triss (and make Geralt choose Triss) since Triss clearly actually loves Geralit, is willing to help him and thinks highly of him, instead of liking the girl who's treats every single person around her like absolute trash (except for Ciri, because Ciri is the daugher she wanted but could never have). The only time Yennefer ever did anything, and I mean anything for Geralt, was when she killed herself trying to save him. And she failed.
Frankly, how you die does not make up for all the pain you caused when you were alive. Yennefer ruined Geralt, that she just barely tried to fix him doesn't make up for it.
Geralt was actually insanely loyal to Yennefer in the books. He cheats on her in the games (mostly because he doesn't know she exists, at least in the first two), but the games are non-canon.
In the books, he outright thinks about how he feels no attraction or love for anyone but Yennefer. It's why it destroyed him when he found out that she loved the mage guy.
He also thinks about how he wouldn't care, as long as she didn't love him. But she did, she admitted that she loved mage guy (even though she clearly fucking didn't, since she abandoned mage guy and Geralt to kill/commit suicide together).
I'm not contesting that Yennefer loves Geralt. She absolutely does. I'm saying she's a horrific piece of shit who acts as an acidic poison to Geralt and everyone who has ever loved her, and that her very touch rapidly siphons happiness and converts it into an even greater amount of assholishness that she will use to fuel her magical powers.
Edit: And Yennefer's tough girl thing is not an act. She actually thinks Geralt is useless, right up until he (spoiler) kills Vilgefortz. You see the scene through her perspective. Bitch right to the end.
Edit2: Nothing to do with anything else, but Regis was really awesome.