The DE is there to give you your ticket. It's yours to lose. You'll make some mistakes, guaranteed. We all did. Remain confident and poised. Make a mistake, move on. Do not let it snowball on you.
If...IF...you bust it, learn from your mistakes, get back in the saddle and take it again as soon as possible. Busting a ride isn't a career ending experience. I had a crusty old DE from hell for my commercial. Before we sat down he was complaining about something or other. My oral was 15 minutes and he said "go do your preflight." I was feeling pretty darn good having "breezed" through the very short oral. We took off and flew to the practice area. That 15 minutes he didn't say one word. He asked me to do a clearing turn and start off with 8's on pylon. I cleared and began the eight. At precisely 180 degrees into the first turn I get, "Ok, we're done here, back to the airport!" I was shocked to say the least. I asked what it was I did wrong and he said "we'll talk on the ground." The return flight was tense at best. We get in the classroom and he writes my pink slip, hands it to me and says, "come back and see me when you're READY," got up and walked out. I sat there numb and stunned, looked down at the slip and fell out of my chair. He didn't say anything about the flight, rather busted me on the oral!!! Lets just finish this by saying after the words I had with him and the flight school, I doubt I would ever be let back in the door much less have him give me a ride again.
Two days later I had a fresh commercial ticket in my hand from a different DE. My oral was about 1-1/2 hours and the checkride a little more than an hour. The guy made it a very relaxing, enjoyable and most important, a learning experience. I slipped up a couple of times on the flight and began to sweat, but he took the time to explain where I made an error and had me do it again. That was nearly 20 years ago and I'm still doing what I love.
2000Flyer