Holding Q will open the radial menu and let you select an item. Let go of it and the selected item will show in your "slotted item" icon. Pressing Q once (without holding it and opening the wheel again) uses whatever item is currently slotted. Potions have a cooldown timer, so you can't use one again immediately .
It's just one of the annoying control schemes ZOS came up with to accommodate the limited trigger availability of console controllers. Much like the access to only 5 skills at once.