Stop feeding your monster.
When something like changing jobs for example, is already seeming huge as it appears on the horizon, the more you think about it the larger the task starts to loom in your mind. And as you speculate about all the possible disastrous outcomes, the size of the task at hand grows with every thought you have. And of course, the bigger it gets, the more you think about it. You keep feeding the monster with your thoughts, it keeps getting fatter. The cycle repeats until the original task has been blown out of all proportion and you've created quite a chunky demon to deal with. So, the trick is to stop feeding the monster (which we should remember doesn't exist) and re-direct your thinking and behaviour. Here's some tips that will help.
Break the task down So, you're feeling overwhelmed. Fair enough. You're not Super-woman/man. But pretending it's not happening/fretting isn't helping. See how you can break the challenge down in to smaller chunks and approach it bit by bit.
What can you do right now? Nevermind what might happen in a few weeks time if certain other things do or don't happen. Recognise what you can actually do right now and if that's nothing, then fine. Have an ice cream and put your feet up.
Write stuff down You might never look at it again but writing stuff down gets the long list out of your head and helps minimise continual 'mind swirling'. This is a technical term.
Talk to someone Share your thoughts with someone who you trust and is happy to listen knowing that you might just need to unload a little. This does not include the elderly lady at the post office who has 'a nice face'.
Start There's nothing like actually doing something to make you feel a bit better. It doesn't need to be a huge step but just something that gives you a sense of progression.