Hi Jack.
My marijuana addiction also was hard to kick. It especially was tough when there was "nothing to do", and I was sitting at home getting antsy and frustrated. Smoking up seemed like a natural fix for boredom....of course, as we all know, as soon as you smoke you just sit there and do absolutely nothing.
Anyway, one key for me was to take those moments where there was "nothing to do" and change my thought process. When you're hitting a lull in the day, and you're at home feeling antsy - GET OUT OF THE HOUSE. Make a plan. It can be something very simple. For example, go to a local cafe and bring your laptop or a book. Grab your camera and hit up that nearby park that you've always wanted to visit. Put on your swimming trunks and hit the beach for a swim.
It doesn't have to be anything major. Just find something that's simple and DO IT. You'll feel silly at first, and your internal addict will tell you it's stupid. Don't listen. Just GET OUT OF THERE and do something. Anything. It works.
PS - stay away from friends who are actively smoking or who might have weed on them. This is not the time for hanging out with that crowd. Good luck.