I'm never motivated to do my sport, and I feel dread for trainings. Should I quit?

I've been swimming since I was 6 or 7, and I'm only a little bit better than others my age. I never feel motivated to train, and I constantly miss practice. I only swim the events I know I can do in meets, and I improve very slowly. I don't get much sleep, and I get slightly above average grades.
Does anyone have advice on motivation and/or when I should quit?