What was your GMAT?
Just my 2 cents (I'm not a pro), I would stick it out if they will consider you a reapplicant anyways. Especially since a withdraw without reason shows poor planning, and you will probably want to address why you withdrew in you optional essay. Just b/c I realized I wasn't up to par isn't a great reason. Good reasons would be a new job offer, new project, promotion, family, etc...
I would start TODAY, on improving yourself for the next year's class. The most important thing as a reapplicant is to show how you've improved, and why you deserve an admit this time. That type of work starts now. I would also retake the GMAT and score in the 80% range, preferably 710+. Then I would apply in R1 or R2 next class year (whenever you app will be the best) b/c these rounds are generally less competative than R3/R4 (if available).
You are correct in thinking that you chances are slim if you are way outside the 80% range, but this doesn't mean you have no chance. You should probably provide the rest of your stats so that you can get the best advice possible including:
Age
Sex
Nationality
School's targeted
Work Experience & # of years
GMAT
Undergrad ranking
Undergrad GPA
Extracurriculars
Leadership roles
etc...
Also, you should ask yourself if you are applying to the correct schools. Once you have improved your profile, you should re-evaluate your school choice and make sure that you are applying to a couple reach schools, a few sweet spots, and a couple safetys. If you apply to all reaches you could be putting yourself in a bad position
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