I once had a young ASB who was so eager to join his friends in the field that he’d take off with me the second I started to close the gate. The next time, I fed him a carrot on his way to the gate. Another carrot before I opened the gate. Another carrot (baby carrots fit nicely in your pocket) the instant we were both inside. Another carrot after I closed the gate. Another carrot before I slid his halter off. Horses love routines, and it only took him three or four days to figure out that if he hung around after I removed his halter, he’d get–another carrot!
As Joe-Joe mentioned, consistency is the key–along with kindness and patience. Good luck to you.