You're doing it for a long time, have you considered starting incorporating some one armed swings? These teach allot since you can't really pull the mace back in with you arms if you under-swing, or stop the mace if you over-swing. You also have to align thing perfectly because you can't correct the balance with the arms either. Actually you can do all this, but it is very hard with a one-handed grip and it's much easier the actually learn to wings better
The one handed grip make the swings very self-limiting.