Thanks for using the forum. I am happy to address your questions, and my answer will be based on the information you provided here. Please make sure you recognize that this forum is for educational purposes only, and it does not substitute for a formal office visit with your doctor.
Without the ability to examine you and obtain a history, I can not tell you what the exact cause of your symptoms is. However I will try to provide you with some useful information.
It sounds like you have suffered a stroke, that was likely caused in part by your diabetes. However, you are still young to have a stroke, despite diabetes, and it is important to exclude a predisposition to clotting, what is termed a "hypercoagulable state" in young people with stroke, even if they have other risk factors for stroke (such as diabetes). This is important because the treatment to prevent further stroke may differ. This can be tested mainly with blood tests. If an inlammatory cause (what is termed a vasculitis) is suspected, then a spinal tap would be needed, but if your blood tests and CTA do not suggest a vasculitis, this would be less likely.
Strokes can cause persistent weakness of a limb, such as your right arm, but physical therapy and exercise is important, and some function of the limb can occur with appropriate physical therapy. Strokes can also cause thinking troubles, and speech troubles, and this would depend in part on where the stroke location was. Speech therapy can be helpful in these circumstances.
Stroke can also cause depression, which can further contribute to thinking troubles and concentration troubles. If there is concern for depression, treatment of it can improve many symptoms.
Seizures or abnormal EEGs after stroke is common. Treatment of the seizures (or severely abnormal EEG patterns) with antiepileptics such as lamictal should help improve some of your symptoms with the thinking. If the neuropsychological evaluation showed specific problems, cognitive rehabilitation to help you with those specific areas may benefit you.
Preventing further strokes is very important; use of a "blood thinner", an anti-platelet agent (such as aspirin or clopidogrel) is important. Also, if you have not had your cholesterol checked, a fasting cholesterol level is very important to have checked, because treatment with specific cholesterol medications can also help prevent future stroke. Aggressive control of diabetes and any hypertension (high blood pressure) will also help prevent further strokes.
Continued follow-up with your neurologist and primary doctor is recommended.
Thank you for this opportunity to answer your questions, I hope you find the information I have provided useful, good luck.