How do I get over the regret feeling?

I know it is stupid, but I love swimming and I was in a swimming gala and had my period so i used a tampon then started getting a stomach ache, I was the best there and was winning every race, now I really regret dropping out because I wanted to show all these people that I was good at something, I was doing 10 races and ended up doing 2. How do I get over it, so I am not regretting it all the time? Please help.