How can I spend the mana from Lion's Eye Diamond?
Lion's Eye Diamond was not errataed to make it harder to use, it was errataed to maintain its original functionality

Lion's Eye Diamond is old.  It was printed in Mirage, which was released in 1996, only three years after Magic was first printed.  As such, it predates the Sixth Edition Rules, the largest rules revamp Magic has ever seen.  The Sixth Editions Rules restructured how a turn works, implemented the stack (replacing a previous concept of spell resolution called "batching"), and notably for our purposes, changed when you were allowed to get mana.

Before the Sixth Edition Rules, you had to have your mana available before you started to cast a spell.  But the Sixth Edition Rules created mana abilities which did not use the stack, and could be used during casting when you are asked to pay for mana (Mana Sources, like Lion's Eye Diamond's ability were not what mana abilities are now.  They acted more like instants that produce mana).  

The intention of Lion's Eye Diamond was to force you to discard your hand before getting the mana, so that you couldn't use the mana to cast spells.  The Sixth Edition Rules made it possible to use LED while a spell was already in the process of being cast, circumventing that intention.  So LED was given errata that required it to be cast at instant speed, returning it to its original functionality.