Its weed. Stop stressing so much about it. I smoked weed with friends in high school, and got caught with it by my school and was suspended for it my senior year. I still got in to a great school, did well, and eventually got a PhD. Its not the end of the world, it is not as bad as people are making it out to be.
If you want to regain their trust the best method is to be open and honest. Tell them you feel terrible for defying them and want to regain their trust. Then, and this is the hard part, make all your decisions with that in mind. This is going to get long but maybe it will help. Aristotle said:
A brave man is not brave once, he is brave over and over again. Bravery is a habit, not an act.
The idea here is that bravery, and in your case, trust, is not about doing one thing. If you want to have their trust, you have to act trustworthy over and over and over again, and eventually, after acting and being trustworthy you will be known as that again. Its a slow process but its doable.