Disability (any condition preventing you from performing well on the job)

You have mentioned in your edit to your question that you suffer from cerebral palsy. I feel that may be an important factor (if not the whole issue, as answered by other posters) to keep in mind in the matter.

I know next to nothing about the condition, but you should ascertain if this has influenced your job performance in any way (e.g. lack of sleep or proper nutrition leading to lack of concentration and/or energy, the need for movement, etc.)

If you can state without doubt that yes, it has influenced your work, you probably need to bring that to the attention of your management. I say PROBABLY because many jurisdictions with liberal labor laws require provisions to be made for people with disabilities (which normally include anything hindering your normal performing of a job, getting promotions, or even getting hired - physical as well as mental - not only e.g. blindness or needing a wheelchair). Even if not a legal requirement, some employers have a more open outlook regarding employees with disabilities. If not, only then it probably won't serve any purpose to bring it up (and that's too bad, but life is not always fair).

Unfortunately many employees are unaware of such legal provisions regarding disabilities, or do not want to "hide behind" something like this. Keep in mind, as a condition acquired at birth, you can't help it, but still need to be employed just like a "healthy" person (probably even more so, due to medical expenses).

If in doubt, it may be worth your while consulting a professional versed in the labor laws in your jurisdiction, e.g. a labor consultant or even a lawyer specializing in labor law. This person should be able to tell you if you would qualify for disability benefits, and how to approach the matter with your employer.

One thing that you need to keep in mind if going this route, is that you should be able to come up with a specific plan how your employer can make changes to accommodate your situation, that will enable you to perform your job to satisfaction. Or at the very least, make a list of specific things that are a hindrance to your performing well and see if your employer is willing to work with you in providing a solution. Let's say someone is bound to a wheelchair, so to accommodate that person, the employer needs to provide access (e.g. ramps) to all areas where the employee needs to be to do his work, adapted toilet stalls, etc. Some person may have a psychological disability where people around her cause anxiety, and so the employer can accommodate her by providing a private office. So think of things that you would need to make your job doable (But be careful to keep it reasonable from the employer's point of view too. I doubt that they will buy you your own solid gold coffee cup when the white china one works as well, to make a stupid example.)

Also, even if/when you get a new job, the same applies and should be addressed from the start, else the problem will just come up again.