You are tense about your erection. Believe it or not, being tense constricts the blood flow to your penis, thus causing the erection to not last long. Drink water (about 8 glasses of 8 ounces a day) to help the volume of liquids in your body. Also building muscle increases your testosterone levels, so start doing push-ups, run, etc. Best of luck!
If you can stop all use of tobacco products, that is a good first move for your health in general. It may take some time for all the bad effects to work out of your system. If you are masturbating excessively, cut it down to allow your body to recuperate. You will have to see how frequently you can do it before it causes erection problems.