Viagra may not be the best course of treatment for you at this time. More importantly, you shouldn't take any type of medication without a doctor's permission. Viagra is prescribed for men diagnosed with circulatory disorders who are usually much older than you. These men experience a weak erection regularly, without regard to sexual partner. In your case, it seems that you have experienced a normal erection with one woman but you didn't experience an erection with your current partner. This is typical and quite normal for men at your age and it is usually a result of anxiety. Additionally, if you take steroids, hard work at the gym might have some influence on your sexual drive. Both you and your girlfriend can try to be patient and tolerant with each other in order to learn more about why an erection is not occuring. You might also seek a diagnosis from a urologist, or mental health professional, if necessary, for an expert opinion.