If you know the legal system well PLEASE HELP ME OUT!?

Well I've had quite a rocky year...I've dealt with major depression (still do) and fixed that with "self-meditating". I've gotten caught with marijuana in april, that was in my shed from LAST october. (I quit smoking in february) And they were bowls. That resulted in consent (step before full probation). Like I said I've been clean since february, but about 3 weeks ago I messed up. I was having a very horrible day, and had that attitude "there's nothing to live for". I got drunk at my ex boyfriends, and ped out there. A couple hours later there were cops trying to wake me up. They made me open my purse, and I had a bottle of pills that contained methadone and seroquel. I was planning on taking them that night, but since I drank I forgot about it. I'm ashamed I even had those on me, because I was doing so well and improving my life. Now I got something in the mail saying if I'm found guilty I get up to 4 years in prison. (For possession of narcotics/cocaine less than 25 grams) - so it says. I realize this is very serious, and I am only 16 years old. But this is what I'm trying to get at: everyone is telling me to plead "not guilty" and it's so frustrating! I don't know much about the legal system but how the hell am I supposed to plead not guilty when I know I am? "Oh the pills just fell from the sky into my purse!" Isn't telling the truth the right thing to do? Well at least I thought so, but in this case apparently not. I might plead not guilty, but I don't know what I'm suppose to say to defend myself? I can't make up a story...that's just wrong. What do people usually do in these cases? I know what I did was completely unethical, and very ignorant. I messed up once and got caught doing it. I just need to know how I can possibly find justice in this situation? Please no lectures...I'm smart enough to know I made a horrible decision... HELP?!