What could it be when the same spot in the anus itches and bleeds for years?

It's hard to tell from your question exactly where in the anal area this spot is. I would recommend you speak to your doctor who can ask more questions, and may know your history, and be examined. This could be a hemorrhoid or anal fissure or just an irritation from wiping (these differences depend on where the area is). Chronic constipation could also lead to all of these issues.