This is how a progression game works. You work hard for loot, so you can be ready to have it replaced by even better loot later. Your work to get the loot was not a waste. Aside from time savings and overall preparedness for new content, endgame content and loot acquisition is the core fun of a game like this.
As I have always said, and will always say - if you did not enjoy progressing your character, you should find something else to do with your time. If you are frustrated, angry, not enjoying the game...stop playing! It's not about the destination, it's about the path you took to get there. If that doesn't sound enjoyable to you, then this game (any loot game, for that matter) isn't for you.