Scared and trapped

I have been at my job four years. I have never been particularly happy there, but I have not let it show in my performance. My family needs the money, as Im the only one working, but also because I try to honor my commitments. I will own to the habit of running a few minutes late, but otherwise I have been flexible and fair.

They have changed my job description and my hours three or four times. Somehow, I always get the extra things that other departments dont want to do. When it happens, they invite me to a managers office and slide a sheet of typed instructions, some words written in red or bolded. Questions are not answered about these changes, Training is spotty in your new responsibilities, but the expectation is for immediate efficacy.

recently, I applied for a different position in the office. Its the next logical step, and not terribly different from what I do now. Including me, there were four applicants, but I was turned down. My boss did not give me a reason that holds any water. He and the lady interviewing me knew full well that I needed this job and the three dollar raise to break even after wage garnishment, but this wasnt mentioned.

I cant afford my medical supplies. If I get fired, my family faces starvation and potential homelessness. I am so afraid to go into work today and face more of the same. Please, can anyone here help me?