With the benefit of a couple decades' perspective, here's what I now might like to say (off the cuff; might write something more detailed later)
It may be so that life's not fair.
But right now, when I hear you say that in response to my upsetness, I feel like I'm not being taken seriously or really being listened to.
I feel like you're trying to avoid being blamed by shifting things onto some nebulous quantity of unfair life.
You may not have an option that could satisfy everyone here. That happens sometimes.
But when I hear you say "life's not fair" I feel like you've given up trying to satisfy me.
What I wish you would say is something like "hm yeah I hear you're really frustrated about this situation, and I'd like to be able to do something that would work better for you, and this is the best I've got right now."
"Life's not fair" sounds like I should just get over it. Like I shouldn't be upset.
What I wish you would say is "yeah, it makes sense that you feel upset if it feels like you're getting ripped off here!"
Maybe you're afraid that if it's okay for me to be upset, that means you've done something wrong.
I'm not seeing it that way.
I just want to feel heard about my experience, and to have a sense that my careabouts are being incorporated into your decision-making.