If you can get one normally, but can't when the "main event" comes around, it's nerves. My boyfriend had a bit of a problem our first time, too. It's totally normal. Once you are comfortable and relaxed, you'll get over it. Focus on how wonderful it is to be with her, not about your performance. Focus on how you feel and get out of your head--stop thinking about it. If the girl loves you, she won't care. She'll understand. Don't think she's judging you.
You'll be fine. Stop putting pressure on yourself or thinking that you have ED. You don't.