The biggest thing you can do is keeping a positive attitude. If what you're called to do is within your ability, everything will come naturally with the right attitude. attitude isn't just "I can do it," though. It's also "I can do better," and "I'll take that criticism and play better" as well as "I have to practice the right way." Keep a good attitude and everything will go fine.
its all about attitude as he said. if u don't have the right attitude then ur not goin to go anywhere. u have to be able to keep urself motivated take criticism and such. if u don't have the right attitude then you're not going to be motivated to practice and therefore will fall behind.
as far as marching and playing
METRONOME i know the eletronic beeping is extremely annoying but those little devices of satan are helpful when it comes to making sure you are in time. another thing would be to learn the music first until u can play it as easy as breathing air and then put ur foot to it...or if ur music isn't broken up into sections already (A B C D..rehersal numbers etc)
u can do it urself and work on marking time to one section at a time and add a new section every time u master one