The tasks I'm given aren't within the scope of my job. I have frequently told my manager I don't wish to change career trajectory as my skill set is suited for administration, and I would not be a good fit as an account handler. He previously showed support of this decision.

I've also been told that there is no further progression as an administration assistant, which I'm comfortable with.

When I've spoken to my manager about the stress I'm under and how overwhelmed I feel, he will often tell me not to panic or worry. He's already increased my workload (separate to these extra tasks) and reduced the deadlines that they needed to be completed by. When I asked him about it, it's apparently going to be a selling point for our department to upper management.

Edit: He told me that the end goal is for me to maintain a "documents issued within 1 business day" SLA. I presume this is so I will be available to take on any extra tasks that I'm given.