Player A
I'm upset with you.
Player B
Yes! You're positively furious! Your knuckles are white!
Player A
You said you were done drinking and I find this?
Player B
That's a bottle! You're angry because you found that bottle I hid!
Player A
You were sober for two years!
Player B
I was sober for two years!
Like its cousin The Waterfall, The Mirror is trying to be supportive but is not taking care themselves. Without giving back, the weight of the scene gets stuck on the back of your partner. And while the The Waterfall comes from enthusiasm, The Mirror comes from fear. They often aren't confident in their choices, or their character so it's easy to just parrot the information that's already out there. It even kind of "sounds right". But it is really hard to play with.
To work on this tendency try exercises they get you out of judging yourself: lights down monologues, aggressive line switching, monologue rants (It's Tuesday!). Or drill the And of Yes And in circles or back and forth. Instilling confidence that they can just go with what comes out of their mouth without thinking about it and that that'll be supported should help break this pattern.