The feeling a person gets when they finish a series Take for example, a person reading a good book series that they enjoy. While they are reading it, they typically look forward to finishing each volume, as well as the whole series. However, when they actually finish the series, they may feel disappointed that there are no more books in the series that they can read. I have observed the same pattern with movie and game series as well.

Is there a name for this feeling of disappointment that comes at the completion of 
a long task? What might a psychologist tell a patient to do if suffering from such symtoms? Can they look at the situation from a different perspective, for example, that would allow them to feel differently about their completion?
I am not sure if there is a specific name for sadness or feelings of emptiness following the completion of a long task, such as finishing the last book in a series. I will need to research this further to see if I can find any studies related to it. I can speak from personal experience and state that I too have felt this, so I believe it is a natural phenomenon. I would be interested to see what others say about it.

Regarding how Psychology would treat an individual complaining of such thoughts, one of the things covered in the Landmark Forum is how to change your conscious, logical thinking as a means of affecting your emotions. As an example, if one felt sadness or loss at the completion of a task, a therapist might encourage the individual to think about the positive: what they accomplished, what they enjoyed during their experience, what they learned or how they grew. By consciously thinking about specific things one can draw emotions connected with those thoughts.

The Wikipedia article on the Landmark Forum describes this as:


  
  There is a big difference between what actually happened in a person’s life and the meaning or interpretation they made up about it
  People create their own meaning to life – none is inherent in the world
  People can “transform” by simply declaring a new way of being instead of trying to change themselves in comparison to the past
  


At it's root, this school of thought comes from the Socratic method, a form of inquiry based around allowing logic to overrule emotion. To address the sadness one feels, one might ask themself a series of questions:


Why am I feeling sad?
What reasons do I have to feel sad?
Haven't I accomplished something good?
Am I not proud of myself for finishing the task?
Did I enjoy doing the task?
Is feeling sad helping me?
Do I want to feel sad?


By using logic in such a fashion, the idea is that one can change their feeling by reinforcing the positive emotions.

Edit: Note that upon further research, I have found that representatives of the Landmark Forum themselves state:


  What we do is personal and professional growth training and development. It's not therapy, it's not psychology, it's not psychological in nature


That's a quote from Deborah Beroset Miller, Director of PR for Landmark Education. Therefore my answer is anecdotal and not actually based on any specific Psychology research that I can find at this point. I will continue to search for actual research to back up my claims...