The key is to track your mistakes and be able to say each time why you were wrong. You should focus currently on accuracy and ignore timing.
The Official Guide is great to practice but not so great to learn from, since the explanations are often very brief.
If you do not want to enroll on a course or get a tutor, I would recommend using the internet and this forum as your tutor.
Apart from that you could use some GMAT handbooks which contain theory and not only practice.