How do you make peace with something that is genuinely unfair but out of your control?

My roommate and I rent a 3-bedroom house that’s attached to a small ADU. There’s no separate water meter, so the water bill is in our name and we pay it. Our property manager gives us a set $50/month credit that’s “supposed” to cover the ADU tenant’s water.

When we moved in, one person lived in the ADU and our water bill was completely manageable. Later on, they moved out and no one else lived there for a couple months, and we averaged around $45/month, so we know that’s how much we typically use. Well, after a couple moved in, it increased to around $102–110/month. So we’re definitely paying for some of their water usage.

I know this isn’t some catastrophic financial problem. We’re probably talking about $10–20/month that we’re paying for someone else’s water. But I cannot let it go. It drives me absolutely crazy.

I’ve brought it up with the property manager multiple times and he offers no help or solutions. My roommate isn’t nearly as bothered by it as I am.

The problem is that I obsess over it. I’ll hear them showering, think about how much water they’re using, mentally calculate the bills, think about how unfair the arrangement is, get really anxious and angry.. Sometimes my heart is genuinely racing and I’ll lie awake thinking about it.

I’m also a freelancer and money is tight, so I know that probably makes everything worse. It’s also just not a fair setup. Why do they get to use as much as they want while we cover the bill? We love living here and don’t want to move out over this. But it’s just so aggravating to me.

I’ve discussed this extensively with my therapist who knows that I have obsessive tendencies. We’ve talked about trying to change how I think about situations I can’t control, but I haven’t figured out how to actually stop mentally engaging with this particular issue.

For people who struggle with obsessive rumination: how do you actually disengage from something that feels genuinely unfair when you know there’s nothing more you can realistically do about it?