Is it considered a mistake to regret not having entered college after performing Istikhara prayer and having a encouraging dream, and then discovering later that the advice which led to not entering was not from trustworthy people? And how can one achieve contentment with Allah's divine decree and destiny in light of this feeling of regret and the belief that the decision was due to a personal error?

The one who seeks guidance (through Istikhara) must proceed with what they sought Allah's guidance for and be content with what Allah decrees. They are not obligated to act upon a dream. If the matter has passed and cannot be rectified, there is no need for sadness or regret. Rather, one should praise Allah for what He has decreed and learn from the lesson. For what Allah has decreed is good and in one's best interest. One must surrender to His judgment, be patient, and seek Allah's help in facing problems.