You've got bigger problems that replacing the mixing valve if your hot is on the right and your cold is on the left. Yes, your mixing valve is failing and you will need to replace it. Do you have an access panel from the back side of the shower? If not, tearing out some tile is in your future. And fix the reversed hot/cold while you are at it.

It sounds to me like you need to replace your valve cartridge. It's really not that difficult to do. Basically, you turn off your water (to the house or the shut-off to the shower if you have one), remove the cover plate, remove the faucet handle, pull out the valve (some have a set screw, some have a set pin to pull, some just screw out), pop the new one in, replace handle and cover, start water and enjoy. Typically the only tool required will be a screwdriver, possibly a wrench.

If you know the brand, you can usually find instructions on the individual manufacturer's website. Valve stems for common valves are readily available at places like Home Depot or Lowes or your local plumbing supply.

Can I pick AIW's answer?
I don't have an access panel. What kind of bigger problems are you talking about? I may not be using the right terminology- the handle rotates from off, through cold to hot in a counterclockwise direction.

Sound like an easy job to replace the valve (probably shouldn't have said that...). I try it this weekend, see how it works.

It is a price pfister. I'm sure they make many types of valves, is there a number of something I need if I go to HD or Lowes?

I agree with AIW as well. I just had this happen, except it was the hot side that was giving no water. In your case it sounds like the cold side is clogged or failed, but it's slightly confusing because your hot side will initially supply water that's been cold in the pipes.

The fixtures were Moen, and I called them even though the fixtures were installed by the builder four years ago, and they are sending a free cartridge! It sounds like some makers will provide lifetime replacement on the valve cartridge. Otherwise it was $39 at Home Depot.

You do need to know what model you have, as there are many different cartridges. I figured out mine by matching up the design on the Moen website.

Oh, and I tried to remove the old cartridge myself using a puller from Home Depot. No luck, so I will have a plumber do it after the part arrives.

Hey, thanks everyone- It worked:) I just unscrewed the handle and wall plate, then removed the piece on the pipe- the valve just slide out. they happened to have the exact on at Lowes for $25. put it back, the water temperature now changes like it's suppose to,

(I'm not sure if I've ever done a repair job with only one hardware store trip)

Glad it worked, although I'm not sure I believe it only took one trip to the hardware store. ;> I've heard of such mythical one-trip projects but never seen one in person. Aren't these tiny little issues supposed to result in a full bathroom remodel?