Not being able to get an erection can be troubling, but you are not alone. For more information on this topic, try reading the answers to Impotence at 15, No Full Erection and Lose My Erection.
The important thing to understand is that erection problems can be caused either by physical or psychological problems. If you can get a full erection when masturbating by yourself, then it is likely your difficulty when you are with a partner is the result of nerves. (FX would like to point out that 13 is rather young to be having sex, and that may have played a role.) If you cannot get a full erection when you are by yourself, the problem could be the result of drug use, diet, or some other physical condition.
We suggest you speak to a doctor for more information. If you do not have a doctor and you live in northern New Jersey, you can call the Adolescent/Young Adult Center for Health at (973) 971.6475 for an appointment. If you live elsewhere, get in touch with your local adolescent health center or clinic.
Please keep in mind that whatever the reason for this problem, it can be treated. And when it is, remember to have safer sex!