I'm doing my employers injustice by being unfit for the job, but what should I do about it?

I suddenly started having seizures and I often get blackouts, even at work or commuting to and from work. The problem is that I have a slight memory problem from having the brain disturbances as well as being a side-effect of the medication I have to take.
I've had this for 3 years now and I still feel like I don't know anything about my job because I keep forgetting things. My employer can't fire me because I'll be protected by Discrimination acts (i.e. can't get fired for being sick) but I feel really bad because they have to keep reminding me what I'm supposed to be doing.
I need this job in order to survive but I can't find a new one because my memory won't allow me to do any better at a new job. I can't quit out of pity for them - I need the money. Doctors won't declare me as unfit for work because they can't find anything in my medical tests to back it up.
What am i supposed to do?

P.S. I don't need tips for memory loss, thanks. I've tried so many already.
I'm just concerned about ethics but more concerned about falling into a bad financial spiral where I won't be able to support myself.

Do what you have to do to take care of yourself. It sounds like the people on your job are helpful. The law is there for a reason and that is to help you. We all bet that "it" isn't going to happen to us but when it does you have the right to do what you need to do to take care of yourself. Take advantage of everything out there that will help you. My brother-in-law went through the system when he got sick. He wasn't sick enough to get disability but he was to sick to be retrained so that he could get a decent job with his disability. He tried to tell them that one of them needed to help him out but they stood steadfast until his doctor gave him some meds that made him absolutely nutty and they had to come and take him away. He got disability because of that trip, not because of his sickness for which he deserved it. He was now certifiably nutty. They system is not reliable even tho it should be there for you. It is admirable that you worry about the moral aspect of not being able to do your job, but you have to eat. Do the best you can at work and that's all anyone can ever ask and that's all they expected when they hired you. Oh and keep pushing the system for help, you deserve it.

I don't know where in the world you are but brush the ethics issue aside and be thankful for the benevolence of your employer for supporting you.
I have no doubt that if he chose he could discharge you on medical grounds as being unfit for the work you were employed to do.
I was dismissed that way and it cost my employer 6 months of my pay, whilst we argued, and a 6 month settlement based pay I had been receiving prior to the suspension and I was working in that job for 6 years.