Made Bathtub Faucet Leak Worse!

I've had a small cold water leak in my tub for a while. I've put off fixing it until the other night when I almost didn't get the water to turn off. I got the replacement valve stem and installed it tonight. It's an old setup and the replacement valve stem is different than what came out. I turned the water back on a now the leak is worse! I've read that it may be the valve seat, but I don't know if I need to reseat it or replace it. Any thoughts on what to try next?

I've included photos of the old and new valve stems and the inside of the pipe.