hmmm... well looking at your piece (and how far along in your development you are).. i think it would be a good idea to rethink the structure of how you have things set up. You have all of your separate buttons, which is good (although, the on(release) method is usually only used on buttons, not movieclip buttons... i'm surprised it works honestly.. i would have thought that flash would throw errors...
) But, as it is, each individual section has it's own "content"(you have your big picture animation within the button). This is fine, but it will create the exact problems you describe, and the only way to fix this is with some sort of function. in pseudo code, it would look something like:
possible buttons = button1, button2, button3, button4, button5 ...
function navigate(what got pressed)
loop through all of the possible 'got pressed'
if what got pressed == button1,2,3,4,5...
then set that button to animate the pressed state
otherwise, put whatever button that didn't get pressed to the off state
and every time you press a button, it would pass that button press into your navigate function... this is a little complex though (especially for your first flash project)... an easier method would be to just place all of your content animations in one movieclip, and then tell the movieclip to go to that frame. take a look at the attached .fla to see what i mean
it takes a little actionscripting, but hopefully produces the effect you want.. write back with questions...