I would suggest that you make the decision on whether to go to series 2 after you have flown again. It is hard to make out from your post why you failed but I will make the asumption that it was not just a silly technical failure that has led to the lesson now being learned the hard way. If you will need remedial training before you fly the test again then do a quick profile and if all goes well then stick with series one; however if you still have errors that would lead to a failing different sections then perhaps you should bite the financial bulet and go for series two and give yourself a more stable run at the test.
Many student pilots end up in this boat and the answer is different for all of them. Perhaps you are down to your last penny, as many end up at this stage, and the thought of more money is weighing heavy on your mind. If so it will be hard to not get overly stressed during the test if you percieve it to not be going well and therefore maybe it would be a good idea for you to go away for a few weeks get some money behind you and go for series two with a less stressed run.
If you choose to take series one attempt two then think of this to keep you calm. The examiner cannot let the flight continue once you have made a mistake that is going to be the reason for the failure; so as long as your hands are on the controls and you are flying then you are on for a pass.