Drug testing - first time

I need some help...

A few years ago I started smoking pot once a day, before bed to help me sleep. I have insane insomnia and have tried everything: sleep aids, sleeping pills, all the 'methods' of relaxing your muscles and nothing seemed to work. It was the only thing that helped me sleep. I'm a small toker, smoking a few puffs a night (1 hitter pipe or bowl). An 8th sack would last me at least a month... so I may puff 2-4 times a night, but I'm really only using a very small amount. It never occurred to me that it would be a problem because where I'm from, Marijuana is decriminalized. But I will be immigrating to the US... where it is not.

Based on the timeline that US immigration has given us, I should be expecting my test in 3-5 months. Not all immigration cases ask for drug tests but there are a percentage of them that do. I have marijuana in my medical history because I experienced a traumatic experience 4 years ago and was given counselling (I had a total of 3 sessions and they cleared me) to which I told them that I had used during that time. If they examine my entire medical history and there is 1 ounce of drugs, they will automatically ask me to do a drug test. Or during my interview, if they ask me if I've ever done drugs in my life and I say "yes, but it's been at least a year", they will still likely ask me to do the drug test.

By the time I do my test, I should be at least clean for 4 months (120 days) if my test is at it's earliest - 3 months. I am female, 120 pounds, 5'3-5'4, and 26 years old.

I'm not even sure what kind of drug tests they'll potentially be giving me... I've read online that it can last in my system for years since I was a chronic smoker up until 1 month ago. I'm terrified of failing this test and delaying our wedding.

Your help is appreciated!