Feeling overwhelm is from knowing HOW MUCH work there is, and/or HOW LONG it must be done for (see the problem with the big picture yet?). To beat this, you have to break things down into bite sized chunks and/or change your perspective on things.
In other words, emphasize the SMALL picture by focusing on doing one small step right away.
Try this: Write down several TINY actions you can do RIGHT NOW. Close your eyes, put your finger on the paper, move it around and stop randomly.
Done? Now DO the action your finger landed on (or closest to).
Think of it like this:
You’re not writing a 10 page essay, you’re just writing for 25 minutes today. You’re not at work for 8 hours, you’re working till you can take your break/lunch.
See what I’m getting at?
You’ve got to not see things beyond the small, that’s what causes anxiety in the first place. Once you can do that, you’ll be able get past this problem.
Don’t make it any more complicated than that: this is about STARTING, not optimizing.