Finally, we get over that useless driving plot of the "reason"(that every readers had realize by now) she quit.S he quit because she is a coward and rather run away than facing the obstacle in her path. It was the same with the violin, she quit because she realize she couldn't be the best and she quit the band because she couldn't win Aki over.
The whole arc is caused by a teenager who is not capable to face her problems head-on and blame it on others rather than thinking how to solver her problems.