I haven't read the other replies, so I am not sure if I am saying the exact same as anyone else, but you have two choices 1)Take your test and see how you do, you might do better than you realize and if you fail, you fail, big deal, just means you will have to pay to retake it again 2) Delay it till your ready.
I was in the same boat as you, I had an instructor for about 30 hours worth of lessons and was a pretty nervous/bad driver, I understood everything, however I just couldn't go a lesson without Stalling twice, holding up traffic, completing a Maneuver successfully on the 5th attempt, I was hopeless and my test was only 2 weeks away by the 30th hour. I changed my instructor, my first instructor was a nice person but really was't helpful, when I did something wrong instead of telling me what I should've done he would just be like "Oh no mate, you can't be doing that" anyways changing instructor was the best decision I ever made, I had to delay my test by a month and whilst I may have failed it first time, I know what I am doing. I can go a whole lesson without stalling, I may do a couple of silly mistakes here and there like going 45 MPH on a national MPH road or drive too close to the curb, but overall I know what I am doing, just to let you know I got two Majors, Stalling on a hill start which made me roll back (The roll back part was the major) and some guy ran out in front of me so the examiner had to use the brakes. Whilst it wasn't my fault, as a driver you have to be prepared to stop.
Anyways I have my next test tomorrow, if I fail, oh well, I know I can drive it's just about driving as safe as possible when it comes to the test. HOPE THIS HELPED!