Ideally first you want a proper grip. Remove the tension from your hands and arms by stretching them. Relax your fingers and shake them to stabilize blood flow. Although not ideally perfect, here's a method on how to find a grip. Take your left hand a place it on the keys you use on Osu. Now observe your right hand and attempt to replicate the general feel and shape of your left. It should feel relaxed and potentially a bit too loose. That's ok it's supposed to be like that. It does not have to look perfect, in fact it's not recommended unless it feels natural. You do not want strain on your grip, your muscles will learn how to aquire it's shape on its' own. Now, place the palm of your right hand onto the middle of the mouse's center of gravity. There probably is errors and left out details in this methodology but I do not care to write in length.
After you have a grip on your mouse, move your hand up-down-left-right and all extremities while fixating your eyes on the cursor. This is important as it helps your brain connect the nerves you are utilizing on your new grip to visual and peripheral.
If you are having a "meltdown" as described, your brain is probably trying to process an overload of information which is fine but as a result, it may temporary toss out some other functions such as grip of mouse and exact precise location of the cursor or circle which leads to you not learning the muscle memory of your right hand. That may be potentially why you describe the word "stroke" om the OP. A stroke may occur under very extreme amounts of mental and physical pressure but of course it doesn't happen often and thankfully if it does happen you do know the roots of what caused the stroke which simply means you just have to relearn the game again... by simply playing with conscious effort. Prioritize your technique and take things at your own pace. I do not clearly understand what causes a seizure but it would make sense for some reason that it may be associated with eyes and brain- your brain is not identifying the shapes and patterns and colors that your eyes see and the overload of pressure results in after imagines to your brain enough to cause the brain to spazz out, or something.
Anyways I'm rambling. There's so much details and potentially mislead information in my writing but I don't want to write in length unless I know there's interest in the other side.
BTW, people that say mouse is inferior or tablet is better simply has not thought about it deeply, or are unable to due to various factors. It's simply the norm to listen what the majority believes and hold it as fact so much as to be vocal about it. There's are benefits and cons to mouse or tablet usage and it's mostly related to learning, not "skill". If you're used to a pen grip like you have your entire school years writing and looking at the time, it might make sense that it appears you do better with a tablet, you know. I have not thought about it too deeply but I believe mouse is superior actually. The grip is more ideal as it intuitively replicates the shape, nerves, and pressures of the other hand. The result of a more even circulation of blood pressure in your entire body results in clearer thinking and focus. Also, I know for a fact from in-game experience that there is unlimited potential in the movement and accuracy and velocity of my mouse. I have also played with mouse and tablet for extended long amounts of time playing maps with of all difficulites which includes 6-8+ ones. I have used tablet with its size as small as possible, and full area, and everything in between. Again, this was not for a short time; for small, for large, etc. I have changed my pen grip a few numerous amounts of time. The inconsistency here is that I change my grip progressively but whatever. For mouse, I have played with speed around 800dpi and I believe 2000dpi. Currently I play with a X/Y axis custom 600/900 windows setting 6. There may be a way for higher velocity in mouse movement related to windows speed, but really I do not want to bother with something that feels near perfect.
Many edits because OCD. Always something more to add on to.