Rely on Who?

The following comes from the commentary of Rabbi Moshe Dovid Valle on Mishle (Proverbs) 3:2.

Many people become full of themselves when they acquire knowledge; they feel they can rely on their own unnderstanding in order to succeed in all areas of lifeBy doing so they lose out because they stop placing their trust in God, the primary source of success without whom success is impossible. This is why Shlomo HaMelech warns us "Trust in God with all your heart, and on your understanding do not rely." There is no bread for the wise, no riches for the understanding and no favor for the knowledgable. Rather, it all comes from God.

Man's understanding is too limited to achieve his destiny. Trust in God with no understanding is more productive than understanding lacking trust. We can see this from King Solomon's own life in which he transgressed the word of Hashem in having too many horses, wives and too much money.

Introspection?

The following comes from Yam HaChochmah, teachings of Rabbi Yitzchok Meir Morgenstern in his explanation of Tanya.
It is damaging for a person to ponder too much about his personal situation. Even though it is proper to examine ones behavior daily to see what behaviors need to be rectified and in what areas he owes thanks to Hashem, still one must remember that God does not come to a person with unreasonable criticisms. It is wrong for a person to live with a constant fear that perhaps his behavior is improper in some way. This attitude flies in the face of knowing that God directs the world. God gave each person unique abilities and situations. Trying to overthink that perhaps you are fooling yourself shows a lack of belief.
Even though you must serve Hashem to the best of your ability, you must also rely on the fact that God is all-merciful and He will help you complete your task. To achieve this understanding truly takes much work.