How can i do better in neet 2018 if i could not qualify this year?

To crack NEET exam make sure all concepts and fundamentals are clear. You can take help of NCERT books for that. Focus on important topics you can also make your notes. You can refer previous year papers and complete mock test series to know your strength and weakness. So, you can work upon them.

You should also learn from your mistakes you have given your paper this year. Analyze your paper why don't you score good marks. work on your weak areas.

Good Luck!!

Go ahead with the coaching. Focus on it because this is the best way to utilize time and efficiency.

Self study is must because then only you will understand where all you lack.
Next thing is revising what your learnt in the coaching and then only you will understand how much clear you are with the concepts.

Give mocks/tests regularly so that you can analyse your weak and strength areas.

A bit dedication and sincerity is required.
Good luck.