How to get in the mindset for becoming a Marine officer?

hey guys im 17 and a senior in high school, and more than anything i want to become an officer in the Marines. I applied for the NROTC scholarship to a college that i really like. but othe rthan that i want to start thinking and getting myself mentally and physically ready. im athletic and enjoy motivating my freinds to do better and keep going, for example gym cl when we work out and it gets tough, i always push them to keep going instead of just stopping, also i push my own self for me, and to motivate them as well but at times, if something gets really tough, and im very uncomfortable, i convince my self to stop, i dont want that whether school work or exercise, what are some ways to keep going when i tell myself "i cant do it" when its my mind telling me and not my actual actions or reality???? thx alot