Not sure why you feel weird about having a hard time with that. It's a violation of trust and a violation of Tara's mind.
We are our memories. I would not be the same person if I one day woke up, not remembering certain fights I had. Small change, big change, it doesn't matter. I would be changed. To have someone I love do that too me because they can't cope with dealing with arguments in a healthy way? I can imagine it, and I hate the idea. It makes me think of rape and brain washing done by cults and that says something.
I could have accepted it if Willow had stopped when Tara said it was a violation and that it reminded her of what Glory did. Before them, she could have been blamed for stupidity, not cruelty. But doing it again? Unacceptable. And still to this day, I don't know if Willow ever regretted it. Did she regret getting caught and becoming addicted to magic? Probably. Does she regret taking the easy way out, ignoring her girlfriends feelings? I hope so.
She used magic to do lots of bad things. The question is, does she regret those things, or using magic to do those things? If magic had been safer to use, would Willow still use her power over people? Was she not only addicted to magic, but to power and control?