The sense we have of incompleteness, what we do not understand or cannot adequately describe.
It is in your writing, projects, and possibly even in people and relationships.
Only you have the answer, and you should be listening to the disquiet of your mind and soul. Feeling like a piece is missing is no way to live for long periods, and while unease can creep in at times, being attentive to these feelings is essential.
When we are stuck, we should put our hand out and look for another to reach in and help, and invite them in.