PB; It sounds as if you should not be fasting at all. There must be some reason for the extreme side effects that you suffer and, as others have said, you should definitely have it checked out. Perhaps you should speak with your Rabbi before YK. He may advise you not to fast until you find out what is causing this problem. You would then be able to attend shul and feel the wonderful awe of the day.
600 kilo bear: you should definitely not be using (or mis-using) Immodium. You could cause a bowel obstruction, particularly if you have ever had any abdominal surgery – even an appendectomy as a child. Did you thoroughly read the label. It is not to be used except for its indicated purpose. Also, using an unapproved (in N. America) medication that you purchase from a possibly questionable source can also be very dangerous. To risk your health in order to have an easier fast is not the right thing to do. If you can’t fast, don’t – but don’t endanger yourself to do it.