I'm feeling those emotions and thoughts too, with an impending change looming. Although it's a beneficial change, my inbuilt fear of change doesn't distinguish between positive and negative, it seems.
What I'm practising at the moment, when the negative thoughts and feelings strike - is to try detach from them and then mentally 'think aloud' and list the positive ways in which this change will impact me. This seems to lighten my mood and dull the sad feelings.