First I want to say that I am sorry that you are in so much pain every day. I can definitely relate, plus I can relate to working full time while in agony. I personally had severe lumbar spinal stenosis (with DDD and multiple disk herniations & bulges). I got to the point that I really couldn't walk anymore. I could only go about 20 to 30 feet before I would totally have to collapse from the pain. I also could not longer walk upright, but stupidly I was stubborn and didn't want to use a cane or walker, though I should have. I've also suffered from horrible pain in my neck and left shoulder, as well as other body parts. I've not had my neck area checked out, but I probably have something similar going on in there as I did in my lumbar spine. During all that, I still worked full-time because I had to support myself. Everyone told me I should be on disability, but that would never pay me enough to live. So what is one to do? We just have to get through each day, minute by minute and use up every single ounce of energy we have to get through work. Then once we get home, we are completely done. So I really want you to know that I sympathize for you and completely understand what you are going through...and you are a real trooper for getting through each day.
Secondly, just as the others posted, you cannot be denied FMLA if your doc filled out the forms correctly and you have a legitimate reason. It is a federal law. You also don't have to take it all at once, it can be taken intermittently or you can take it as a reduced work schedule..all for a total up to 12 weeks. What your employer can do is to request a 2nd medical certification from another doctor if they really wanted to (like a 2nd opinion) but they cannot deny you if you have just cause. FMLA is enforced by the Wage and Hour Division of the U.S. Dept. of Labor's Employment Standards Administration and you can bring civil action against your employer for denying you FMLA leave (of course you need all your proof, etc. and having a lawyer would probably be a good idea as well). Maybe you can call this Federal Department if need be.
Also...I'm sorry that I don't know your history, are you currently seeing pain management at all? Or is your current doctor trying to work with you to try to control your pain at all?