Okay, I'm an idiot. (no surprise)

This is the shower valve in question (Matco-Norca). I'm in a condo that was built 3 years ago, so the valve is "new"

This piece attaches to another piece which attaches to the metal pipes which gives my shower water.

I took it apart because I wasn't getting any shower output below a certain temperature. I put it back together and got my shower working at any temperature... But now it leaks between the red ring and the hub seat (I'm using this diagram - it's not the exact system but it looks the same to me).

I've tried magic putty, silicone, teflon tape, and I'm not sure what to do. I'm using lots of pressure to tighten, so much so that when I overtighten it it hard to turn the shower handle.

What makes it even funnier, is that I have a second shower in my condo with the exact system, I took that apart, and in putting it back together it too leaks between the red part and the hub. It makes me feel like I'm putting it together wrong.

All the seals look okay - no corrosion (when I touch them they look/feel in excellent condition).

Please help before I pay $500 in idiot taxes. Thanks