How old are you?
1. Find out if your GP has made any record of what you told him. If it ain't in writing, it didn't happen, and unless he prescribed something (Valium, Zoloft?) he may not have recorded anything. Besides you might have simply been fantasising and he didn't believe you.
Furthermore, if you only did something once or twice, it would not necessarily be material in my opinion. I'd discuss it with your GP in more detail including whether he thought it was material and discussing exactly how anyone would find out different from what you say on your medical form if there is no record.
2. But there is a deeper issue, there is a difference between being "young and stupid" and using drugs as self medication for personal problems. If the latter is the case, then you had better think very hard about whether those problems still exist, because they WILL resurface during your training when it becomes stressful, as it will no doubt be from time to time, and if they resurface and you resort to self medication again, then you and anyone flying with you, are in deep deep lethal trouble.
To put that another way, if you lose situational awareness while flying on instruments in a dark and stormy night, from stupid drug or anxiety related stuff going on in your head, then you are simply dead.
3. Thanks to the random drug testing regime, you will get caught if you do stupid stuff. Not sure if usage two months ago will be picked up.
4. It is not unusual to be mixed up and unsure of what you want to do with your life at your age. Exactly how did you determine you wanted to be a pilot? What aircraft have you flown? When did you decide this? Last week? Yesterday? Do you know what the job really entails?
If you haven't dipped your toe in the water and tested your aptitude, then maybe now is the time to do so before you commit yourself (and your parents hard earned cash) to training schemes.
To put it another way "Not so fast young Skywalker."
On a brighter note, I'm afraid I've had to deal with similar young peoples problems, and the individuals concerned now hold some pretty responsible qualifications in a transport environment and are doing just fine.
To put it another way if you've grown out of it, and it was just "youthful high spirits", and there is no record, forget about it, don't mention it because it's not material.
OK,now flame me for potentially encouraging an illegal act.