While some people enjoy doing nothing so much, some others experience a condition which in some cases can be worse than anxiety when you have a lot of work to do. Some people find it very difficult to stay without doing something, especially when their daily life is related to work and only work. The condition that occurs when you have nothing to do can get worse if you call yourself a " workaholic ".
If the anxiety and nervousness you experience in such situations seems not very manageable, you should consult a mental health specialist, as this condition is likely to be a consequence of a more important cause. On the contrary, you are simply a productive person who feels empty when you do nothing.
Do not stress. We know, it sounds like a cliché, but it matters a lot. The best thing you can do is to stay focused on what comes next. Think that the day will come when you will not have time for anything, so do what you can to enjoy it.
Set tasks for yourself. In order to keep yourself busy, start planning future projects or jobs. Make a list that you can use for later.
Deal with small things that you know how to do well. It can be house cleaning, arranging shelves, folding clothes, writing poems, screenplays, lyrics, video editing, etc. You know best what you know how to do brilliantly. So do it.
Start something new. If the periods when you have nothing to do are long, why not start something new? Do you have any hobbies that attract you? Any sport? Think about things that interest you and deal with them.
Help someone else. If you do not have things to do, give a hand to the people around you. You will surely find someone who needs your help.
Improvisation. Take a walk. Read a book. Watch a movie. Cook a dessert. Call a friend. Go drink something alone.