You can find the entire shower valve assembly at your local big box for around $40.00. I must tell you that this is not a temperature regulating valve. It is simply the original shower valve mechanism.
Cut a hole in the wall behind the shower valve coming in from the other side of the wall.
Then here is what you do:
o turn off the water;
o cut the old pipes;
o remove the assembly;
o fit the new assembly in place; and
o solder all the necessary piping back together.
After checking for leaks, replace the drywall that you took out to get to the assembly.
An Important Note: I made sure there is sufficient support for the new shower valve. Also made sure there is extra support for the tub spout.
A very common problem is some people think this is a foot rest. For example, while shaving their legs or just relaxing in the tub. It is not for that purpose. So, reinforce accordingly.
Just a Thought: You have to admit, replacing the drywall from the rear to fit a leaky faucet is much easier than placing new tile on the wall or having to replace a fiberglass tub enclosure.
As far as the time and effort expended in this case, replacement of the valve assembly is faster and cheaper than the time you would waste trying to fix the old leaky faucet valve and it is far more reliable.
The really difficult part of this project is getting the right parts. I always carry the old parts to the big box and try to match them. Most faucets have a life expectancy of, at least, 10 years. However, vendor product lines get updated regularly.