There is a latent period between two erection which is a gap of few hours to many hours

Once you get used to masturbation, the mind gets  used to getting stimulated by the illusions you build up to get an erection to masturbate. You will not get an erection easily when you face the real thing(gf). When you have a gf, why masturbation? give it up.

You dont need any medication, because this is the way it happens. Your sexual system takes some time time to get ready for another intercourse after an ejaculation. This period is called refractory period. It ranges from a couple of hours (as in your case) to a few days (in the case of middle aged and the elderly) to get another strong and long enough erection after the ejaculation.

You must understand that excess of everything is bad. Same is true with masturbation. Control masturbition. Dont do more than a once in a week otherwise it may cause some weakness. You may take cap. Shilajit & tab. Chandraprabha bhati. One tab. Each morning & evening & also take dhatu paushtik churan one spoon morning & evening. All 3 with hot milk for 3-4 months.