She did lose someone, in a way. She was very close to Hawkeye, and they kind of drifted apart when he went rogue. When she meets him in the alley, she tells him something the following:
Clint Barton: Don't give me hope.
Natasha Romanoff: I'm sorry I couldn't give it to you sooner.
meaning she tried (but likely failed) to appease him, and herself in the process.
And more than that, while she's being all workaholic, she also (like every other hero) lost half of the world. Thanos handed their butts to them, even when they all teamed up, Gods, guys in suits, superpowered individuals - and still they lost. And after that, when Thanos destroyed the Stones, there was no hope of everything, there was just no point.
After all the path she had walked in previous MCU movies, overcoming her doubts of not being an actual hero but a "monster" (because she had killed people), that double loss had a terrible toll on her - that's why her sacrifice for the Soul Stone is the end of her arc, the culmination of it, with her being an actual hero.