Terrible constipation for quite some time. can't go unless use mag citrate or enemas. now have small black stool with mucus. colonoscopy on wed?

Right idea; see GI: You could have Constipation predominant IBS or an inflammatory problem of the bowel causing possible blood and/or mucus to appear. Let a Gastroenterologist (GI) interview you and assess your risk factors. It sound like a Colonoscopy is a good idea, but that test should be done once it is clear what diagnoses you are trying to rule out (especially at your young age). Good luck.