One method is to push back on work. It's not easy:
"Sorry - I can't give you any delivery date for [x] as I'm working flat-out on [y]" or
"No, it will not be ready for Friday, as [x] is taking priority"
Standard boss response: "It has to be!"
Next up is negotiation: "If you need that, which of these gets lower priority?". Push the decision back on them. They will probably be reluctant, so give them an out by suggesting they go to their bosses and ask for a priority.
Do you have any job planning software? If not, even just post-its on a board can work as long as it's very visible. Put time realistic estimations on the post-its, and arrange them to show when jobs are going to finish with big lines at week or month intervals, and as they add more everyone can see the effects, pushing the others down.
Lastly (as a nuclear option) consider mentioning that you are feeling very stressed because of being given too many jobs with no priority planning. Having a key member of staff signed off for two weeks from stress may motivate them to plan better.
Do be aware that this is relatively normal. You will always be pushed to do more, in less time and with less resources. You have to learn how deal with it.