Which part of a guitar solo do you find hardest to play?
Which question in a job interview do you usually find hardest to answer?
What kind of algorithm do you often struggle with?
Which chapter in a good book did you find hardest to read?
What part of your exercise routine do you find to be the toughest?
When you are practicing, those are the parts to drill.
The parts that feel most uncomfortable is usually the parts we often skip. Yet, those are the parts where the potential for improvement is the greatest.