hi Andrea, so glad you found the board. Your BF has had a shock and is now full of good intentions. He gave you a peep into his reasoning when he told you he never thought you'd break up with him. Now he thinks it's finished; he's sorry, he will fix his drug problem, he appreciates what he had - why aren't you on board? Because you know this is just a beginning. He may not realise it, but your instincts are right.
Remorse and good intentions fade, and then the hard slogging work begins. The moods, the sleeplessness, the voice telling you you weren't that bad, it's under control now, you can go back to just using a little. Your BF hasn't even got to that bit yet.
So stay careful. Give him a year or so; you'll probably know by then how serious he is. By all means meet him and explain your decision, and don't let him guilt you into letting him back when you know his default position is to take what he has for granted.