Sometimes if you give it a couple of shots with the WD-40 and wait, it will come loose. Also, penetrating oil (watch you don't drip it all over and stain things). A torch sometimes will cause enough expansion (doesn't take much) to allow you to break the corrosion and get it loose. You need to be careful with the torch! as you don't want to blister the chrome or any other parts. Often the serrations on the stem are rounded off and a new handle will not resolve the problem. If the handle insides have been rounded off, then a new handle should work, if not, then it is a waste of time - you'll need to replace the valve or the stem. If you can't get the handle off, you should be able to pull the entire handle, stem off and take that to a good plumbing supply house and get a replacement stem. Then, your new handle will fit! Maybe it is time to replace it with a new pressure balance valve (and or a temperature controlled one). From an advanced hobbiest.