The FL never never truly loved the ML, and thus she will not get hurt that the ML has other women. On the other hand (I personally think that the ML loves her but is just denying it), the ML clearly has more affections for her than she has for him, and is stuck in a lie that FL really loves him, so he's the pitiful one in the situation. We would expect the FL to be suffering to see how the ML sleeps with other women, but she never does because she isn't in love with him. This is EXACTLY how a transmigrated female from modern times who know how emperors have many wives should behave. She knows clearly that she wouldn't be the ML's only woman, so she never invested true feelings, and this makes the ending satisfying as we see her live peacefully and carefree in the palace for the rest of her life, which was precisely what she planned from the start. From all the other concubines and even the empress, our FL has the highest position in our ML's heart. Despite not being the empress, she's still the most influential female in the palace.

As mentioned in the novel, "The job of a concubine, can be prioritised but you can't catch feelings. For if you catch feelings, you'll be subjected to eternal damnation." Eternal damnation of heartbreak, is precisely what the FL managed to avoid by treating her time spent with the emperor as a necessary job.