try playing each piece measure making sure you start and end on the left foot (that includes the release) if you have trouble with that break it down beat by beat
write out where each foot hits the ground in relation to what your playing on your sheet music

You'll get it if you practice but what helped me my freshman year was I practiced just marking time or just marching alone until it was second nature while thinking or listening to a copy of the show music (or you could do whatever you want to be able to march to) and then practice the music until it is second nature and once you can basically do both of those things asleep then you shouldn't have a problem putting them together but you should play it chunk by chunk to be positive you are on the right foot. Keep at it man, no worries.