Feeling empty after a deadline

I submitted my PhD ten days ago and since I've been feeling completely empty. I have a small kid and a secure job which takes a few days of my time, but not any new interesting project coming. This situation is giving me a strange form of anxiety, I know I should relax and enjoy a quieter moment (I have always worked a lot) but this doesn't seem to work, I feel unable to concentrate and fear of being stuck. I also get plenty of fears about the future and get insomnia. I had suffered from depression before and so I fear it would relapse.

What would you do? How can I be productive and happier in this situation? What has helped you in a similar situation? Thanks for your advice.