Can you ask for / take a break before you see her again? I don't know if that will work in your situation -- if it does, I'd think it would be ideal to give yourself a bit of space to deal with the heat of immediately having to decide between wanting to not hurt her and yet making your feelings known.

But, yeah, after the break, I'd go with what others have said about not forgetting what she's done.

I haven't faced this problem with a T but I've repeatedly run into this with ex-partners / 'friends' / family I've been hurt by -- what's usually helped me (and this may have limited mileage for you) is to think through the consequences of each type of action I could take.

Usually, given my past interactions with the person, I'll have a pretty good idea of how things will shake out. So, then I've decided what I can live with based on -- a) what I want my relationship with the other person to ultimately shake out to be (or if I don't want a relationship at all); and b) my values / principles.

Sometimes this has meant just keeping quiet and not getting involved at all because that's been my most 'authentic' response and I believed it would help me in detaching in the long run.

And, at other times, it's meant that I've been involved / helped in a very limited capacity after having decided on my boundaries (which I wouldn't necessarily disclose to the other person) and sticking to it.

Of course, this hasn't made it any less painful, not to mention that it has sucked up a huge amount of cognitive resources. And, of course, brought on a ton of negative responses from the other person (usually not the most stable people) -- so, knowing that I decided on what to do in advance helped somewhat to stand my ground.