Masturbating frequently is only a problem if it interferes with your life. Like if you do that rather than go to class, or go out with your friends, or meet girls so you can have a 2 person sexual relationship. If the rest of your life is fine and satisfying to you, don't worry about how much you are masturbating.

Stress could be part of your problem. Also, maybe you are doing it too much and aren't getting as excited. Try backing off for a couple of days and see if the desire comes back. Guys need new images sometimes. Variety. Find new pictures, get new fantasies, etc. Just don't stress too much about it unless it becomes a serious problem and you can't get excited at all. Then you may need to get a physical to make sure everything is working right. Otherwise, it's probably all mental that is easily fixed. Not physical.

hey thanks for ur reply,i m geting excited but it is taking me a lot longer,my penis remains erect for a shorter time,sometimes i also masturbate lying face down,my friends told me it is ok unless it does not hurt you,could that be a problem?

Maybe, if you are pushing on it too much, that could make you loose your erection.