I was having normal erections. after having sex for the first time in 18 months, twice, i have not been able to really achieve and especially maintain one since. its been 2 days?

Analysis of data on young men presenting to a urologist with ED, ages 14 to 35 years, found that 74% of these men coming to major medical centers had a physical problem, and 60% of those had an underlying vascular problem. The rest had a variety of causes including psychological. You should see your Dr for a thorough exam and potentially a referral to a urologist. Early treatment is vital.