I use to mastrubrate every day from the age of 18 and some times 3 times a day and my sperm was dark and thick and at the age of 30 the sperm became thin and at the age of 31 one day I mastrubrate 4 time in 1 day back to back and with pressure on penis then after that i feel no sensation  in my penis from last 15 days and I am unable to mastrubrate with excitement and found it difficult because penis is not standing in full strength and after completion of mastrubation penis squeeze very faster. HOWEVER I have shown to urologist and he had the test all was normal in blood, urine and ultrasound test. So please help me and can anyone tell me what has happen to me and can this be cured by medicines or from surgery. Which type of doctors can help me please tell/suggest