Is there any way to help me reach the Bari-Lower notes such as D,Eb,C,C#,B,Bb,A and still have enough breath to move to the next note.

it Just takes practice ull get it

if all you`re doing is hitting an 8th note, then try and hit it staccato, should save some air; and yeah, just keep practicing, get up some stamina, or if you`re like, 15, growing will help with your lung capacity.