Marines but now I am second guessing?

I am 17, 18 next month and for the last 4-5 years i have anted to join the Marines and now that I finally figured out that I was joining the Marines after my first year of college, I am starting to second guess myself. I made up my mind about joining about 2-3 months ago. I have not signed anything and seen a recruiter once but is this normal? Am i just second guessing myself because it is going off into the unknown? Or should I go back and reconsider the decision to join. I know for a fact if I dont join I will look back at it later and life and regret not joining. Thanks for your time.