Yeah, the limited spell slots are straight up just a mechanic from D&D, so that's something you'll find in all CRPGs using 2nd and 3rd edition. It gets easier at higher levels, as you have more spell slots, but in BG at low levels, you do just have to rest often. It is fine to have wizards using, say, a sling for the easy fights and save their spells for when they really need them. If it helps, though, with most quests there's no actual time pressure, so you can't fail it if you do have to do one fight, then rest, then the next fight, and rest again. If you like turn-based, BG and BG2 have settings for auto-pausing with each turn, which replicates a turn-based system by allowing you more time to plan out your next move.