If you cast an enchantment spell during a turn before Psemilla is on the battlefield, its first ability won't trigger that turn even if you cast another enchantment spell later in the turn.

Psemilla's last ability checks at the moment it would trigger to see if you control five or more enchantments. If you don't, the ability won't trigger at all. If it does trigger, the ability will check again as it tries to resolve. If you don't control five or more enchantments at that time, the ability won't resolve and none of its effects will happen. If you do, it doesn't matter what happens to those enchantments later in the turn.