What should I do about my job situation?

I’ve been with my company for many years and was thrown (not really asked) to work two departments, I was not being compensated btw. One of the departments is very difficult and there isn’t really a “back up” person, I was getting very frustrated and finally spoke up about it and I was given a raise. But after years of doing this I find myself not happy having to work both departments anymore I get very frustrated, there isn’t good communication and I’m treated like a door mat. I wanted to speak to my boss about being removed from one of the departments but last time I tried that I was told that they could not move me bc they don’t have a person to replace me. I care for my job and I don’t want to quit I just don’t want to work both anymore. I want to feel fulfilled and happy about going to work not like I’m being dragged to my doom. I’m not sure how to go about this.