I am often overthinking things aswell - I recommend trying to focus on what is happening right now. Start counting the things on your desk or concentrate on steady breathing, this often helps 
Forbearance, it means that you are patient in doing everything, do not be squeamish, do it with a normal and reasonable rhythm. Let it flow, let it run as it is, if you are squeaky, then you will feel anxious and restless. In the end you will think negatively, and mess up everything you do, and you will bother to check that you do many times.