Sense of Accomplishment in Meditation Regardless of a meditation's perceived quality, I feel that finishing a meditation provides some sense of accomplishment.

Nowadays, I am experiencing great lack of motivation, and when I set the goal of meditation as '30 minutes', I stop half-way and feel that I failed. 

Knowing how finishing things provides accomplishment, would it make more sense setting shorter times which I can finish? Is there such a thing as the momentum of accomplishment in meditation or even in virtuous activity in general? Or, is such a feeling of success a kind of attachment, and problematic? 

Thank you
Here are some excellent advice given by Ajaan Fuang in "Awareness Itself":


  § "When the meditation goes well, don't get excited. When it doesn't
  go well, don't get depressed. Simply be observant to see why it's
  good, why it's bad. If you can be observant like this, it won't be
  long before your meditation becomes a skill."
  
  § A young nurse practiced meditation with Ajaan Fuang several days
  running, and finally asked him one day, "Why wasn't today's session as
  good as yesterday's?"
  
  He answered: "Meditating is like wearing clothes. Today you wear
  white, tomorrow red, yellow, blue, whatever. You have to keep
  changing. You can't wear the same set of clothes all the time. So
  whatever color you're wearing, just be aware of it. Don't get
  depressed or excited about it."
  
  § A few months later the same nurse was sitting in meditation when a
  sense of peace and clarity in her mind became so intense that she felt
  she would never have a bad mood infiltrate her mind again. But sure
  enough, bad moods eventually came back as before. When she mentioned
  this to Ajaan Fuang, he said, "Looking after the mind is like raising
  a child. There will have to be bad days along with the good. If you
  want only the good, you're in for trouble. So you have to play
  neutral: Don't fall in with the good or the bad."
  
  § A student came to complain to Ajaan Fuang that she had been
  meditating for years, and still hadn't gotten anything out of it. His
  immediate response: "You don't meditate to 'get' anything. You
  meditate to let go."
  
  § "Don't make a journal of your meditation experiences. If you do,
  you'll start meditating in order to have this or that thing happen, so
  that you can write it down in your journal. And as a result, you'll
  end up with nothing but the things you've fabricated."
  
  § "You can't plan the way your practice is going to go. The mind has
  its own steps and stages, and you have to let the practice follow in
  line with them. That's the only way you'll get genuine results.
  Otherwise you'll turn into a half-baked arahant."
  
  § "When the mind's not quiet — that's when its poor and burdened with
  difficulties. It takes molehills and turns them into mountains. But
  when the mind is quiet, there's no suffering, because there's nothing
  at all. No mountains at all. When there's a lot to the mind, it's
  simply a lot of defilement, making it suffer."


Maybe you can give yourself the flexibility of meditating between 15 to 30 minutes. At the end of it, don't cling to any result/ state, and be observant about why today's session was the way it was. If today's session was not good, it's not because you had failed. It was probably due to some defilement entering the mind. Then that would lead you to identify how the defilement came into the mind. That's more skillful than blaming yourself.