Psychological stress, performance anxiety, depression, neurogenic disorders peyronie's disease, body image issues, medications, alcohol, smoking, drugs/medications, hormonal imbalance, radiation therapy, kidney failure
, metabolic disorders, multiple sclerosis, hypertension, diabetes, prostatitis, urethritis etc. just to name a few. Without a definite diagnosis at hand it is difficult to suggest management plan. I would suggest considering a detailed evaluation by a urologist.