i have to agree with all of the comments mentioned above, it would be a good idea to get some professional guidance and a check up if for not other reason that to reassure you that this is not your problem and that you are fine and that everything is as it should be and working.

Often the anxiety of worrying about such things can be the reason they exist, you should try to relax more and let your body react, maybe an idea to play by yourself more, to experiment what works for you, how to maximise your own pleasure, and turn your body on so to speak, there is no user manual for the human body, we are all pretty much unique.

this is however not your problem, and you are in no way to blame, i agree with whipper, a period of abstinence usually has the opposite effect, you are not being stimulated correctly or adequately, hence your bodies natural actions are not performing accordingly, this is it in a nutshell.

forget cumming on demand or how instructed, learn what your body needs, go play, get yourself relaxed and enjoy yourself most importantly.

It is your body, go have some fun with it. i bet that the more you think about this the harder it gets, difficult i know to say but relaxing and letting yourself go is key here, hence if you can maybe investigate and explore when you feel more relaxed and on your own it would help greatly.

Good luck, and last word i promise there is nothing wrong with you honestly *smiles*