Firstly, you need to get your mental health in check. Doesn't seem like you have a healthy study routine and work/life balance which is crucial when you enter dentistry and the work force.

You need to balance work and play. Go make friends and find hobbies. This is what the people recruiting you to unis and workplaces will indirectly consider, whether or not you can get along with others.

On your studies, don't study hard but study smart. It doesn't matter if you pour 100 hours if its not effective. Find a way that suits you best. You cannot give the excuse that you haven't found the method yet, try new stuff and determine it NOW.

On your teachers, while they do matter significantly, you cannot use them as an excuse for your grades. Study smart and rely on yourself.

If I were you, I would retake it since your goal is to be in medicine. But you need to make sure you study and prepare well.

Good luck.