just my personal interpretation, but based off of how she has to reaffirm herself multiple times about it, she probably wasn't truly happy with her life. i think she was content and fine with how things were, but she also knew that mayumi would always be more "free" than her in a sense, therefore her happiness would always be "truer." it's not that her entire life was a waste, more so that there was just a completely different life she could've lived which may or may not have granted her more fulfillment
The ending with her saying she”ll wait even decades for the call from her lover from so long ago sums it all up. Man I love Yuri but this authors works piss me off so much😭