This must be a record of some kind, you have tripper across 4 bugs in a single sketch. I finally late last night figured out what was wrong. As well as the ones already listed the LIPO batteries are configured incorrectly and won’t connect to the breadboard without a wire (which of course due to the previous bug is fatal!) So here is a new copy of the battery as well as the others and a sketch which is complete and I think correct (I corrected the level shifting resistor voltage divider which is incorrect in your original). With that I don’t see any issues that should burn out voltage regulators, and motor back emf is unlikely as that should be suppressed by the L298D. So the cause of the burn outs is still a mystery.
A new LIPO batery that connects to breadboard without needing a wire:
LIPO-2000mAh-fixed.fzpz (6.6 KB)
and the fixed up sketch (other than pcb which I ignored), it has the new parts in its temp parts bin so you don’t need to download them. You can either export them to save them outside of Fritzing (which is what I do in case of database corruption) or just drag them to the mine parts bin to reuse them in other projects.
fritz_arducar-fixed.fzz (87.9 KB)
Peter