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:
Work Experience & # of years
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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