I don't matter at work but can't afford pay cut

I need leave my job because it's physically very strenuous. They will randomly increase my work load when they feel like it. I am never really allowed to leave when I am technically supposed to. I am constantly told I am less than everyone even though I worked here a year. I constantly am training new people and I finding it hard to hold my tongue. They hired a bunch of new people and the new people come in expecting a lot and I feel bad because I can't crush Thier spirits without revealing I hate my job. I physically been getting really sick and do not think I can physically do this job anymore but I don't make enough at my other job that's 35hours a week. I also can't complain without risking getting fired. What do I do?;