There are basically three ways an engine's going to die.
1. Clearances inside will get so large that too much oil is being burned, and it's fouling sparkplugs and catalytic converters, making the engine un-usable.
2. Something breaks or starts grinding itself to bits. This usually happens because number 1 has gone on too long, and the engine is starved for oil. It could also happen from an engine that was pushed beyond its design limits (i.e. too much or incorrect modification).
3. It overheats and a head or the block crack or warp beyond repair.
Of the three engines I've actually declared "dead" over the years, I had exactly one of all three. One was a neglected old Buick that the prior owner hadn't changed the oil on enough, and it was not worth saving because it wasn't a very good engine to start with. The next was another Buick that broke the crankshaft because I had pushed it too hard, for too long, making too much power. The third was a terrible old Monza/Vega engine that overheated when the radiator blew open, and the aluminum block warped. Again, the engine sucked, and was junk the day it left GM, but this just confirmed that.
From the sounds of your oil usage, you're looking at the possibility of number 1. In any case, keep changing the oil and don't just add. Acids and junk build up in the crankcase from combustion blow by, and you need to get that out. Just adding more oil doesn't get that corrosive junk out.
