>What Shinji needs is to love himself, accept himself. Not seek validation from others.
Except Kaworu represents just that. Shinji didn't really start questioning himself until he was forced to kill Kaworu himself, which was the final straw that sent him into a downward spiral of true self-discovery.

Keep in mind that throughout the series, Shinji's constantly trying impose himself as someone he really isn't. And it never ends up working for him in the long run, or for those around him.
Every time he engages in masculine bravado, it backfires immensely, only bringing more inner pain to him or those around him.
His hobbies, interests, mannerisms and overall demeanor are far more effeminate than they are masculine. He's the one happily doing all of the cooking and cleaning in the Katsuragi household. Not Asuka, not Misato, just him.
And throughout the entire series, the audience is led to believe that Shinji will end up falling for either Asuka or Rei, only to get duped once Kaworu shows up.
Meanwhile Shinji grows increasingly distant with his so-called female love interests and colleagues as things turn from grim to worse, his views on women in general becoming further driven by disdain, rejection and outright objectification. Because he was still latching onto this false sense of self that he's imposed on himself, and on others.
Shinji feared women because they remind him too much of himself. The inner self that others mocked him for. The inner self that he inwardly hated, ignored and rejected for so many years.

Hell, Shinji was originally planned to be a cute tomboyish girl until Anno randomly decided to change the gender all because he was tired of doing standard female protagonists.

The celesta thing was a good choice, feels like the game!