My manager always asks me to cover other people’s shifts

Okay I need some advice, I have 3 days off from work, I have been looking forward to these 3 days off for the past week, I’m so tired and exhausted from work and just life responsibilities outside of work, my manager has text me asking me to come in at 6:30am tomorrow…

I feel sick, because I struggle with saying no, but I physically can’t do it, I’m so tired, I’m so fed up of this job, I’m always asked to come in.

On the other hand I feel so guilty, because a coworker is sick ( mind you it’s one who constantly talks shit about me) I still feel bad, but is it really my problem? My work has recently lost 3 people, and hasn’t made a serious effort to employ anyone else as of yet, we are understaffed, and where I work if one person calls in sick, it’s a shit show…

My manager is manipulative and not very compassionate, doesn’t care about people’s situations much, I’m finding this job is consuming my life, but I need the money whilst I finish my education in order to get a job I’m passionate about.

What do I do? Should I feel guilty to say no? I am so sick of being asked to come in!