I would suggest you start as soon as you can. Remember, reviews are very quick so you don't have to become a slave to it.

You must ask yourself why you failed:

Lack of knowledge: Assume you know nothing and start revising from the begining and follow the method I suggested.

Unable to apply knowledge: If you feel your knowledge was acceptable but you were unable to cross-reference it or apply it to the questions then only review the material the keep it fresh, but spend your time practicing the examples for real. This ecourages you to remember how the knowledge fits together as part of the DBA job, rather than a string of facts.

Exam technique: If you feel you failed because you were nervous or your exam technique was bad then consider buying STS and sit mock exams. Try to structure them like real exams with the same timescales and restrictions. This extra familiarity will reduce nerves in the real exam. Don't start taking mock exams until you feel you know all the work. The main thing is to be honest with yourself. It's no point blaming exam nerves if the nerves were induced by the fact you did not revise properly etc.

It's like anything in life, you have to put in the effort. It's easy to convince yourself you've tried hard when actually you spent your time staring into space. Over time the process will become normal to you and learning will be easy, sometimes even fun

Cheers

PS. Remember, it's only my opinion!