Shower leaking

Hi,
My cold side in the shower is leaking pretty bad, I replaced the gerber rubber washer but still leaking, I suspect the seat might be cracked but with the special tool to remove and replace with a new seat the inside is stripped too bad to unsrew.. any suggestions on how to approach this?

Tks

Some times the seat is just rough, they make a tool called a valve seat reamer that will help, but i've never had much luck with one other than to stop the seat from eating up a new washer. If the seat is cracked then it has to come out, try some PB Blaster and push one the tool real hard and turn, then you can try other screw drivers or even a sharp wood chisel. You might end up replacing the faucet, but you have nothing to lose at that point, so try drastic measures, like the chisel tap it lightly with a hammer and push real hard while you turn, but a couple coats of PB Blaster or better if you can find it Liquid Torch first. Some times ust the right size screw driver will tap in there and get it out.

Be sure you have the right rubber washer for the valve, if it is not right it won't work. They come in all shapes and sizes.