I'm on SSDI for my epilepsy. For me it wasn't necessarily difficult but it was definitely a process and took about 18mos-2yrs. You will almost always get denied the first time. Then you get a GOOD LAWYER who specializes in SSDI to file your appeal and the waiting game begins. It helps if your doctors support your claim and make sure you have copies of ALL of your medical files. I can't emphasize GOOD SUPPORTIVE LAWYER enough though, my lawyer was key to my approval.
I should also add that I was very pigheaded about going on SSDI. I had to be talked into it by my psychologist...I thought it was charity and that i would be taking from someone who really needed it.