I just left my fiancee of 3 +years and I'm terrified he's going to turn back to drugs.
Are you concerned about what might happen if you do?
I'm afraid that he might relapse, I'm afraid I can't trust him with drugs
It's understandable that you are concerned because of his prior habits. You should communicate with him about your concerns. This will help you to avoid worrying and it will let him know that someone cares.
I've met another guy, and I want to start a new relationship as soon as possible.
I can understand how you might feel. You should communicate with someone you feel concerned about. You can express your feelings with his family so they can look in on him.