The first step is to quit worrying. Relax. It's supposed to be fun, not a job to do.
Rule out certain factors. Medical anomalies such as ED may not apply to you because of your age. It's most likely a lack of experience. The more you do it, the longer the session is.
Understand physiological differences between men and women. Your triggers may be more physical whereas a woman's could be more emotional. In other words, she can last longer than you. So, you have to figure out how to delay your response while satisfying her prolonged enjoyment. Foreplay may be your asset.
Using a variety of techniques, such as stimulus aids with "toys" for her which include phallus products, and for you, rings, (look it up) can prolong the relief you're looking for.