The best answer I can give is to masturbate till orgasm, ejaculation. I have this happen sometimes- yes it's almost painful. Usually, my morning erections are manageable. Sometimes they last a long time, and I'm in my 50's. During puberty, I had to masturbate, ejaculate. People do understand this too, if they can see you still have an erection under your clothes. It's common, and means you're healthy.