Dear
Welcome to HCM
We understand your concerns
I went through your details. First of all you should understand
masturbation is normal and it is not unhealthy. Earlier, when you were young (you are still young), you use to have hard erection for almost twenty minutes. Now a days your erection is not that firm and you ejaculate within around 5 minutes and your erection is lost.
Here, many things could be at work. If I say this is normal, you may not believe. But that is fact. Your age, sexual interest, sexual fantasies, frequency of masturbation, engagement with other aspect of life like education career etc, determines your erection and ejaculation. When you grow older, your mind do not stick to sex alone. Don't worry. Enjoy life, sex, education and career. Don't be anxious.
If you require more of my help in this aspect, please use this URL. http://goo.gl/aYW2pR. Make sure that you include every minute details possible.
Hope this answers your query. Available for further clarifications.
Good luck. Take care.