My manager has beef with my colleague and is assigning their work to me

My manager has a habit of being unregulated which allows their emotions to supersede their professionalism and objective thought. In their latest spat, they are upset with my colleague for a variety of reasons, which is now resulting in the manager assigning my colleague's work to me. My workload is already bursting at the seams.

We're a two man crew, yet my manager wants to be the center of attention throughout the entire organization that consists of over a dozen different functions. She seems to want her hands in every department's operations, so she takes on anything and everything with no discernment of whether it's possible or how easy it will be. So that is why our workloads are immense and approaching our limits quickly

But now with her petty grievances with my colleague, my workload is continuing to grow to a point that I have way too much on my plate to be able to focus on anything consistently enough. This low pay government job is already starting to creep into my evenings and my weekends and I'm not okay with this.

How should I approach this tactfully? I'm at my wit's end and I'm about to quit.