Fish balls aren't bouncy and fall apart easily
The most likely cause was this part:


  slammed it probably around 100 times in there, then added a tablespoon of corn starch (with a little water), mixed it in, shaped a fishball, and put it into some very lightly boiling water.


You need to blend the water with the fish paste thoroughly. What I do is I dump all ingredients into the food processor and blend for about eight to ten minutes, excluding the time taken to scrape down the sides, or until the paste becomes a little elastic. When you push it with a finger tip and release, there is a little resistance. 

There is no need to slam the fish paste at all.

For bounciness, add either baking powder or egg white. In my case, I add 1g of baking powder per 100g of fish. Alternatively, 0.4g of baking powder and 5g of egg white.

Also, add at least 44-50g of ice-cold water and 1g of potato starch. Commercial fish balls contain even more water for more bounciness.