It is much easier to say, You neither overestimated nor underestimated your ability. In others words, you made a good estimate of your abilities! Hope this helps

The way you wrote the phrase was awkward for two reasons. You used a double negative in the first clause (‘NOT only didN’T you underestimate’ or ‘NOT only did NOT you underestimate’). In the second clause, you used the past perfect tense (‘did you overestimate’) instead of the past tense (‘you overestimated’). A better way to write this phrase would be: Not only did you underestimate yourself, but you actually overestimated yourself.

It is understandable, but I would suggest changing it to: You didn't only underestimate yourself, but you also overestimated yourself. You could also shorten it to: You not only underestimated yourself, you overestimated yourself.

"You should neither underestimate yourself nor overestimate yourself." sounds better. Also, it could be "Not only did you not underestimate yourself, but you also did not overestimate yourself." But pass tense is weird here.

Hey! It is not correct. You should say: ''Not only did you underestimate yourself, but you also overestimated yourself.'' Hope that helps!