Just as some people refuse to see their faults, so there are those who insist on digging too deep, persecuting themselves over every fault and making unreasonable demands upon their lives. Eventually they collapse from exhaustion, or worse, kick back with resentment.
This is why no person should go it alone. Everyone needs a mentor, someone who can look objectively and say, “This is where you are right now. This is what you can expect from yourself right now.”
See Hayom Yom, 26 Cheshvan; 20 Kislev 5737.
Pushing yourself... In the well-known Greek myth of Icarus, he is warned not to fly too close to the sun, for his mechanical wax wings will melt, or too low, for his feathers will clump in the sea's spray. Similarly, the Jewish concept of humility...an accurate assessment of oneself...fits with this concept of pushing oneself as much as we were created to do...but no more. Aiming to achieve more than G-d intends is hubris and ends in destruction. Aiming to achieve less is irresponsible/lazy, and also ends in destruction. Each of us need G-ds help to continually soar at the right altitude. This is at the heart of the Rebbe's sentiment.