Talk to your DM about changing the duration of certain spells

Mage armour lasts 8 hours, the same length of time a long rest takes, because it needs to last for the full adventuring day without being able to be cast before the start of a long rest so you can effectively cast it without a spell slot. It should last 3 days with this system.

Other 8 hour spells should be similarly changed, and hour spells should probably last a day or 8 hours. Obviously 1 minute spells are meant to last 1 combat, and combat hasn’t changed length, so they don’t change.

If your DM says no to this he doesn’t understand the point of gritty realism and just hates casters, switch to a fighter.