How to complain professionally?

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I was hired initially to help a more senior accountant because she was overworked. For 6 months, we've been working on the same project. Initially, she would assign things for me to do but that was just fine because I saw it as a learning stage. Eventually, we split the project 50-50. Although she was still the more senior person, I didn't have to account for myself to her. I report to the manager and to the controller. A few months ago, the manager saw it fit that I get projects of my own. That meant I would spend less time on the shared project to focus more on my own projects. The thing is, I still take 50% of the shared project and worse, the senior accountant chose what jobs to dump on me, which are the more time-consuming mundane tasks. On top of these, I have my own projects to finish. I don't mind taking on more tasks but I see her bumming around and complaining about tasks she used to do. She claims she cannot handle the shared project on her own so she still needs me. But she volunteers for additional work and uses this as an excuse to dump more work on me. I want to complain and say that I don't want to work with her anymore. I don't mind taking on the shared project on top of mine as long as I don't have to partner with her anymore. How do I say this without sounding like I'm complaining?