Basic helpful information for you. Your penis gets erected through increase of blood flow, which the medula in the brain helps activate once it is excited. Once you "tend" to it, and satisfing it, it continues to stay up. So you feel you must again to make it relax, and my guess is that is why you feel you masterbate too much. Toprol XL is a widly used drug in many hospitals, and it is a blood pressure reducer. However, keep in mind, by doing this might lead to erectile difficulties later. Go to your Doctor and ask him to do a Cholesterol lvl, and a Lipid Profile if he sees you with a High Blood Pressure danger!. If your not, you most likely don't want to be taking that, cause then when your not having a constant erection, your blood pressure might be by far way to low. Which can give light headed ness, and can lead to sleep apnea i believe. All you need is to relax a bit! After ejaculation sex stays on your mind. A great thing to try is Melatonin. Melatonin is a natrual enzyme that reacts with the body to force relaxation...Like the scent of Vinilla. Great way to get the enzyme naturally is through Milk. If you get it through milk take and warm it up to get the enzyme more reactive.
Also, do you have alot of stess in your life? It's possible your overly stressed, and rather then some men with erection difficulty you might react differently. And if you are overly stressed, might want to look into an anti-depressent.
Many men look into sexual release as a stress releaver, you might still have that stress and strive for the need of release, and so it stays up until then. Just a possible thought