Should I quit my job?

So far my part time co worker has taken 4 weeks of holiday. And i have covered an equivalent of 10 days of mc for him in a span of less than 3 months, and yet he has been still scheduled in the shifts more than mine. Even tho i feel theres bound to be a shift id have to cover for him again, should I just quit instead of questioning it with the manager ? I feel like my work performance isnt very good either so itd be a rocky thing to ask.