What do you do when its all feeling too much?

Lots of things going on lately, plus one very large work assignment. Some days its all easily handled, but on others, there are so many extra things going on that just overwhelm my senses and I find myself full of anxiety. Sometimes I find that just waiting for the right time works, but on others it seems to become more and more elusive as time goes on. Anyone have any advice for how to deal with the more challenging days?