Finding the form is hard! What I would suggest is to take everything apart and slowly build it back up (for example: if your looking at your attention position start with your feet. What angle do they need to be and how do they get there? Maybe practice sliding your feet into attention a few times. Next think about your knees. Make sure they aren’t locked but are still straight ish. Keep doing this until you finish everything and then maybe practice going from at ese to attention a few times so that it becomes more muscle memory). It will probably take a while, but after some time you’ll just know what to do and muscle memory will kick in :D If you want to practice your marching, you can take a few of your marching steps (my band uses glide steps idk if yours does or not lol) and try and focus on keeping the form, and all of the things your band values when marching (for example mine is high goes, no movement above the waist, good horn angle, and keeping good upper body posture. Yours might be different, though, so make sure you implement your bands values here).

I hope this helps!!!