Thinking things are impossible means you already have failed before you have tried. Believe in the impossible. Even if you fail to achieve it, you will likely go further than you would have had you believe it was impossible.
Don't let you thoughts invade your soul. Remember, they are just passing through and do not define the inner you.
I know I've posted this one before - but I think it's a good reminder. Over thinking injects more into situations than is really there. It makes mountains out of molehills. Don't do it. Or resist if you can.
You control how you feel. Just as you can overthink a situation for the negative, you can talk yourself into a different way of feeling by how you think. Choose the positive.