Thalia had been left behind.
Everyone had abandoned her.
First, it had been her father. Even though Zeus was around in later years, he just saw her as an asset and had abandoned her as a father long before she was born.
Then, it was her mother. She had abandoned Thalia when she found out that Zeus had left her. She stopped caring for Thalia and her brother, thus abandoning them to the cruel world around them.
Her brother had abandoned her not once, but three times. Even though it wasn't his fault, he abandoned her at the Wolf House when he was young. Then, when they had found each other again, he abandoned her when he went off to the quest of the seven. After they all came back, Thalia got to spend several years with Jason while he married Piper and had a happy life, only for him to abandon her again when he died of cancer.
Luke's abandonment probably hurt her most of all, even though it was the most expected. He had abandoned her when he poisoned her tree, even though he was her closest and best friend, right from the start.
Percy's abandonment was totally not his fault. And she didn't blame him for it. He had died in a monster attack at the age of 30 and it had hurt everyone. Even though he didn't do it on purpose, she still considered his death an abandonment, after all, he didn't even say goodbye.
Nico took the longest. After his husband died, he was so distraught that he wasn't seen for years, but he eventually tracked her down so that they could reminisce because we were all cousins. He lived to the age of 105 and died in his sleep, thusly abandoning her, taking away one of her best and longest friends.
Annabeth had probably abandoned her the most. First, when she had woken up from her long slumber in her tree, Annabeth had been captured and taken away from her. Then, she abandoned her to go on the quest of the seven and fell into Tarturus. After that, she abandoned Thalia when she left Camp Half-Blood to live with her girlfriend Reyna in New Rome. Lastly, Annabeth had abandoned her when she had died after she was hit by a car.
The hunters had not abandoned her. She had abandoned them. After 1,200 of being in the Hunters of Artemis, she had quit. She had become tired of the constant moving around and of all of the hunters only slightly tolerating her because she had become more and more temperamental as the years had passed.
Now, Thalia stood before her window in her apartment in New York and wondered, Why was I abandoned by the ones I loved? Was I not good enough?
And then she jumped.