From the song "Sick Boy" by The Chainsmokers, which quote or lyric from the song would best go with the idea of struggling with self-identity?

The core of this question involves a "What do you think?" scenario. Different listeners are going to gravitate toward certain lyrics more than other lyrics in regards to this particular question. This song is really quite an amazing song considering the genre (EDM). Often this genre produces music that is thematically about love or being a strong individual. Take Martin Jensen's "Solo Dance" as an example.

I came to party on my ownDon't need nobody in my zoneI get down to the beat, I lose controlEh oh, I go so so-lo

Sophie Elise's "Love Like That" works too.

If you close the door you better never come back, noDon't leave me with a love like that, love like that, love like that....

I suppose a very outmoded sort of argument could be made that a major difference between those two examples and "Sick Boy" is that those two songs feature female vocalists, so of course they will be about love; however, that kind of argument doesn't hold up. An easy example of a male EDM vocalist singing about love is Kygo's "Firestone."

Our hearts are likeFirestonesAnd when they strikeWe feel the loveSparks will flyThey ignite our bonesAnd when they strikeWe light up the world!

In general, EDM music is very positive thematically. It sounds happy and generally is about happy things, so "Sick Boy" is a notably different kind of song. It very effectively is calling out a common societal problem. That problem is narcissism.

Welcome to the narcissismWhere we're united under our indifference

The dictionary defines a narcissist as someone that is extremely self-centered and has an exaggerated sense of self-importance. In other words, a person is infatuated with himself or herself. Many social media posts support this narcissistic argument because many of the posts essentially say "look at me" or "look at what I'm doing." Further evidence is just how many selfies abound throughout all kinds of social media apps.
"Sick Boy" attempts to tell listeners to stop believing all of the projected greatness that is out there when it says to not "believe the narcissism," but that's something that society is struggling with. Many studies are now emerging that link social media to depression. It's an inverse relationship. The greater the time spent on social media, the less happy people generally feel. The depression could be considered a prison of sorts, or the constant need to present your best self on social media could be seen as a prison as well. Again, the song points this out.

Make no mistake, I live in a prisonThat I built myself, it is my religion

As for which lyric best supports the idea of a person struggling with self-identity, I think it comes fairly late in the song; however, it is repeated four times.

How many likes is my life worth?

This line clearly shows how a person can become trapped in finding personal self-worth through the number of times people click "like." Even someone as controversial as Kanye West has picked up on the dangers of the "like" button. He questioned a possible link between too few likes and suicide. Not enough research exists yet to fully answer that question; however, it does show how a person ties their self-worth and their personal identity to what other people think of them online. The lyric in the "Sick Boy" song links a person's identity to an arbitrary number of clicks. That is a sad and unfortunate thing, but I believe the lyric is clearly pointing out that a person who is so dependent on those likes is clearly struggling with personal self-identity and self-confidence.
https://www.livescience.com/62718-social-media-habits-depression.html

https://www.nbcnews.com/news/us-news/social-media-contributing-rising-teen-suicide-rate-n812426