Chronic anal itching, burning, sharp

I'm feeling itching, burning and sharp pain exactly inside the anal rim during/after defecation and during/after walking. This is happening since last 3.5 years. The problem is intermittent, i.e., stays for few days and goes away on its own. Sometimes it's very savere, making walking and defecating extremely painful. I've visited multiple doctors; gone through a surgery to remove anal fissure and hemorrhoids, tried sitz bath, laxatives, oitments, dietary supplements and multivitamins(all through prescription), still no luck. PS:  I don't have constipation and I follow healthy diet and lifestyle. I'll appreciate any guidance regarding diagnosis or treatment of the problem, because it has become chronic and the treatments/doctors couldn't solve it till now.