For context my manager is going on an extended leave and I have stepped up to cover their role. Which is fine and something I wanted to do. I am not being compensated for it but there has been discussions of a promotion following all this goes well but nothing guaranteed. Now my gripe is its our busiest time of year (I work agency and have one client) however my team members are on leave for 2 weeks which has been approved despite it being our busiest and we are having major shift with my manager being away. My cover will only have started which means we don't have experienced hands on deck.I have flagged this as a risk and built plans outlining where additional help is needed and I’ve been told after asking for help (we have a lot of resource) that “oh you can manage this can’t” I said “how is it possible for me to cover 2 peoples role?”And I was met with well you have to if you want to prove yourself and get somewhere. I am worried about burnout and just not being able to manage doing 2 roles they say for 2 weeks but then in the xmas holidays and we have people off and it will be me picking up all the slack. Surely that can’t be fair. How do I professionally fight this. If they expect me to literally break my back for a “potential promotion” nothing in writing. I am feeling really hurt and used at this moment and it seems no one is considering my wellbeing.