I understand how you feel. You didn't meet expectations set by people in school and your family and I'll be honest with you. I have had examinations in the past where I clearly didn't live up to the standards of scholars, especially in year 7 for me. What did I do? Took a week off and seriously, just went into holiday mode. Took a break, had fun and played games and went to Hong Kong completely forgetting about work for a while.
Let me ask you this question. Did you give 100% in that 3U exam? If you did, you shouldn't feel ashamed. Unnecessary worrying over an exam will not alter your mark so it's better you choose to forget it and continue on. When you look back at that exam paper in a decade's time, you'll probably have a laugh and simply leave it aside or chuck it in the bin.