anyone came and asked me to do something that wouldn't allow me to complete other tasks, it was boss' decision to say which of the tasks will be dropped/delayed to next week.
I just learned a phrase for this! "Only so many penguins can fit on the island." So now whenever news tasks get added to my workload I picture them as penguins zooming up out of the water, and then I picture penguins falling off the other side of the island.