I'm not going to cover for my coworker today
Why won't you cover for your co-worker today?
I've been covering for him and it's really putting a strain on me.
You should be helpful to your coworker. They may have been mean to you in the past, but you should help them out.
I've covered for him too many times in the past and I'm done with it.
It sounds like your co-worker has some issues. Explain to him why you won't cover for him and encourage him to focus on performing better at work. He could lose his job if he doesn't improve.