You only have to do one thing at a time. In fact, you can only do one thing at a time. (That thing you call multi-tasking? It’s just switching between several things very fast and not properly concentrating on any of them. Ask me how I know.)
So there’s a big thing you need to do? A job you can’t imagine how you’re going to finish (or even start)? Something you said ‘yes’ to before you worked out how you were going to do it? Good, welcome to the freelance world. Say yes first, work out the details later.
In fact, the best way to work out the details is to do the thinking as you go along. Begin. Open the file, take out your pen, pick up the phone – there must be one thing you can do. Write one word. Read one document. Then do the next thing.
Sometimes simply beginning is the hardest part. And the only way to get over that is to do it. Quick, before you come up with another excuse to put it off!