In Witcher 2, Oils and Potions would last X amount of minutes, a set duration. In Witcher 3, they changed it so oils have charges that are used up when you hit and potions had their effective duration toned down.
Witcher 3 then gives you back the effectiveness of these potions in ranks. Regular, Enhanced, Superior as you level up. The problem, is that the game is at its hardest at the early levels (-15) because your weapons, items, bombs, poitions, oils and such are at their base stats. Yeah, I was cursing the potitions, foods, oils and other for being so useless, bu now that I'm level 21, everything has peter'd out.
Game gets easier as you level up and become beastly through the advancement of your gear and items.
Hang in there.