First go at growing my own food failed miserably

I’m excited to get into permaculture but it’s very intimidating and I know nothing about gardening so I decided to choose something simple – potatoes. I grew them in bags and followed a youtube tutorial but the yield was pitiful, the only potatoes I got were less than half the size of the seed potatoes. Feeling discouraged and wondering if anyone might know what was going on:

Details:
– I used 5 bags, they were fairly big, but only 2 actually produced potatoes, and they were very small
– All plants grew quite large.
– I added soil when the plants reached 15cm in height
– I found curl grubs in only one bug Idk if that’s bad, but that doesn’t explain the other two that didn’t produce
– I tried using my wee as fertilizer for some bags and not others, the 2 that produced I didn’t fertilize but they had smaller plants which is weird, I read too much fertilizer can prevent tuber forming, but that doesn’t explain why there were so few potatoes and they were all so small
– I had loose soil but in one of the bags soil there was a lot of mycelium growth which made it more firm, but this was actually one of the bags which produced.
– I followed guidelines for when to plant and waited a bit over 3 months to harvest, some of the seed potatoes had started to rot and of the newly grown potatoes there were a couple which had a chunk missing which makes me think something was eating them, but it’s weird that there were only 2 like this and every other potato was whole and the bags that didn’t produce literally had 0.

Did I miss something obvious or does anyone have any ideas or suggestions? am still keen to experiment and dive further into the world of permaculture.