How can I keep from feeling dishearted while trying to live mindfully I find it very difficult. I get bored sometimes and other times simply struggle to stay focused on the breath and the body. Does my attention span improve over time? I'm ADHD so it's very difficult to keep thinking out. Is it ok to think?
Thoughts are generally wholesome and unwholesome, so yes, everything might be a tool for achieving your happiness. Just be mindful that this is a wholesome thought when it arises.

In the same way, achieving bliss cannot be done without utilising Self in the "desirous" (but not thrisna here) way to let go of Selfishness. In a way that confidence should trump doubt. Such is the way.

Also, what does it actually mean right mindfulness:


  "One is mindful to abandon wrong view &amp; to enter &amp; remain in right
  view: This is one's right mindfulness...
  
  "One is mindful to abandon wrong resolve &amp; to enter &amp; remain in right
  resolve: This is one's right mindfulness...
  
  "One is mindful to abandon wrong speech &amp; to enter &amp; remain in right
  speech: This is one's right mindfulness...
  
  "One is mindful to abandon wrong action &amp; to enter &amp; remain in right
  action: This is one's right mindfulness...
  
  "One is mindful to abandon wrong livelihood &amp; to enter &amp; remain in
  right livelihood: This is one's right mindfulness..."


From Right Mindfulness: samma sati 

You are simply aware of the body and breath in order to recollect you to keep the right view and refrain from cultivating the wrong view. It is a "reminder" that is always available at hand (breath). It also calms the mind giving clarity.

You also probably try too hard since Middle way should mean Right effort as described above. It should bring joy and peace. I agree, it is not easy to keep balance but you know when you find it.