You need to prepare before you can start killing monsters. It’s not just about throwing on the best equipment you’ve got and running in to battle. Geralt is a skilled alchemist if he’s got the materials and formulae to work with, but more importantly, he can meditate.
Meditation is one of Geralt’s most important abilities. Doing this allows you to skip ahead a set amount of time, changing the time of day and restoring any Vitality he has lost. While meditating, Geralt automatically replenishes any prepared potions and oils as long as he has strong alcohol in his inventory.