Basically she betrayed Kirari, and when she realized what she'd done, and Kirari was still respecting what she did, but she's like "I'm worthless".
How did she betray kirari
Sayaka basically tried to controlling the votes so they all go to help Kirari Momobami, but that went against what Kirari believes in, so she felt like she failed but Kirari is amused while Sayaka doesn’t see as a success in her part. (Sorry if this doesn’t explain it )
Spoilers ahead for people reading the comments before reading the manga
Sayaka gave money to the house pets to buy their chips to help win Kirari. When Kirari won the seat (again), Yumeko (iirc it was her) explained that the money given to the house pets was enough for them to be relieved from being house pets. That devastated Sayaka because that was against what Kirari had built so she broke down thinking she broke Kirari's precious aquarium. Kirari didn't mind though (even offered her blazer to Sayaka when she broke down) but still Sayaka's mental state wasn't good and still isn't because of what she did to her dear president
BRO WHAT HAPPENED TO SAYAKA, I'M SOOO CONFUSED