Assistant, I need your help. My manager is overworking me. Help me IM them something so that they get off my back.
These situations are best resolved in face-to-face conversations, which allow for more holistic and direct communication. However, in some cases, it is difficult to find the right time to discuss such a sensitive topic, and it may be better to communicate one's concerns earlier rather than later. Here is a possible instant message you may want to consider sending to your manager:

Hey [Manager's Name], I wanted to flag an issue regarding my current workload. I feel like it's been increasing recently, to a point where I can no longer ensure that I meet our quality standards and/or the deadlines. While I'm dedicated to my role and always strive to deliver my best, I feel that the current workload is unsustainable.

Could we perhaps set some time to chat to go over my workload, reprioritize, and perhaps delegate some of my tasks to other teammates? Thanks!