You've heard the question - "How do you eat an elephant? - Answer: "One bite at a time".
This picture which I painted recently made me think about biting off more than you can chew - or to reframe it - stretching yourself to take on something that at first sight seems too big.  
I know it sometimes seems daunting to look at a big challenge and think to yourself that it is just too big a stretch to take on.  But if you break it down into small bite sized chunks, it can be achieved. You've probably heard the saying, too:
"The journey of 1000 miles starts with a single step".  It's true. You can tackle something huge by just making a start - doing a little at a time, and plugging away at it until you are suddenly over the hill and coasting down towards achievement.  It helps to imagine what it will feel like to achieve that goal.  It is said that we are more likely to achieve things that we write down. So set yourself a task, write down what you are going to do, and by when. Make a plan.  Tick things off as you achieve them.
Believe that you can achieve the impossible!