Single Scope Background Investigation and the USMC?

When I was about 16yrs old, I dabble with drug culture a good bit. For a few months i smoked marijuana everyday and even tried psilocybin mushrooms. At this time of my life I did not think that I would ever come to consider joining the Marine Corps. Now I am 18, much more mature and haunted by the error of my ways. I enlisted into the Marine Corps and am trying to be a Signal Intelligence analyst under MOS 26. Not until recently did I know that I would have to undergo a Single Scope Background Investigation as well as a polygraph test. Not only am i scared about that but when I enlisted I didn't tell the recruiters the whole truth about my run in with drugs when I was younger and am scared that I'll get busted for the inconsistency in my stories as well as lying. Any advice would be highly appreciate. I really screwed up and hopefully I can leave my past behind me once and for all.