This happens: suddenly you're feeling totally overwhelmed with the things you still have to do. Especially when responsibilities change.
Sometimes it can be really productive to just go at it and work as hard as you can to make everyone happy. And this can be a great solution, because there are times when you just have to do the work.
However, if there are really big shifts in priorities or in the amount of stuff that is on your plate, it can be helpful to reevaluate your areas of responsibility. Start by making a list of all the big things you are (or feel) responsible for. Now, try to sort them, based on everything you know.
If you work alone, just chop off the bottom 90%, so you are left with the top 2 or 3. Focus on those areas. If you're working with others, ask them for input to make sure you sorted them correctly. This is a huge help for all new requests coming in: if they don't fall in your list of priorities, your default answer should be no.