You're not a mean mummy! Maybe he just needs you there for a while. Once his routine is re-established, it might get easier. As you said, you were gone for a month, completely changed everything he knew and he's grown up a lot in that time. It must be very frustrating for you, but I think, if you work with his needs, encourage him and keep reassuring him, he'll eventually learn that you are still there for him even if you're not in the room ITMS?
That's what I'm hoping, fingers crossed.