This is particularly the case with children. As a teacher I have often wondered why the 'tunes' in their early stage books were so short (4 or 8 bars long). They gradually increase in length as the pieces get harder. This early-stage-brevity is actually a good thing: otherwise they would be quite overwhelmed with a 32 bar piece, if they are only 7 years old, and have only been playing a few months.
Our ability to concentrate will normally increase as we commit to regular practice. This increase in concentration is remarked upon by some Primary School teachers when they know that one of their pupils is receiving lessons outside the school.
So, be encouraged! Regular practice is good for us!