It's common to fail a station. It doesn't make you a bad EMT; you probably just overlooked something. I failed two my first try. Here are a few things to keep in mind:
Though it's not testing your ability to use a BVM technically, and it's a bit of a juggling act using a BVM for 1-person CPR, make sure you're making a good "c-e" seal with a good head tilt, chin lift with adequate rise and fall of the chest. Also, when the AED says "do not touch the patient," make sure that your second rescuer doesn't touch the patient while it's analyzing. Make sure your arms are locked and you're getting adequate depth and recoil of the chest with each compression. You can continue compressions while the AED is charging, but make sure to clear the patient before delivering a shock. Turn on the AED, then apply pads when directed.
The only other thing is be sure to look over your critical fails. Were you nervous? There's always "failure to manage patient as a competent EMT," that's pretty much as broad as the horizon. Additionally, there are some good videos on YouTube that are good for review.
As far as re-takes, you test once, then can re-test same-day if you fail 3 or fewer stations. If you fail them again, you can register to re-take the practical stations you missed for two more sessions, two times each test day. If you fail, then I think you have to retake the whole practical at that point, but I doubt you'll get to that point. Good luck!