I don't blame you for getting mad. It is unbelievable how unfair things are at work. And I think they don't appreciate you because they already have you, but they idealize a new junior employee because they haven't fought with them yet.

What used to happen to me was that companies would see specialized skills on my resume and hire me for entry level jobs with low pay. Then they would demand that I take over graphic design and computer work so they can stop paying expensive contracted workers to do those jobs. A lot of employers suck and I'm totally pissed off with the way you are being treated.

It is really depressing when you can't find a new job and they know you are stuck. Have you considered leaving a review on glassdoor.com about your experience in the company? It might not do much but sometimes negative feedback makes me feel a little better.

I hope you will eventually find a better job. They will be in trouble when you leave because they rely on you so much now. I once quit a job without telling a supervisor a password for a thing they used every day that I created/updated. They took it for granted that it would always work until the new year rolled around and no one knew how to change the year. By then I was far away and didn't reply to their email. That was my tiny revenge.

Hang in there. It's infuriating being treated like that especially when you know your own worth.