I am a newbie and still grasping in understanding BTS. Afaik, members has gone through a lot. I read a lot of answers in this question was Yoongi who has the darkest past. But, some members also have past / life that were not easy either.
These are what i get in past few months since i start my journey knowing BTS:
RM, had depression in 2014 – 2016 and the height was in 2015 – early 2016. It was because he was the most hated member of BTS, at that time. As a leader, he hold a lot of responsibilities. He is the face of BTS so people would attack him first. People called him for cultural appreciation, mysoginy, plagiarism, etc. Even he was called as the ugliest and untalented (one of “fans” said it right in front of his face and he just answered ‘i know’). His facial feature and natural tan skin tone didn’t meet with Korean beauty standard. Plus, underground hip hop singers (esp. B Free) called him out because he chose to debut as an idol.
He was also “attacked” in a variety show (it is really popular show until now) because of his clothes or his hair colour.
“Fans” and haters said a lot of cruel things to him. Even said to him to kill himself because he was ugly. He also got a lot of dead threats. When read a lot of bad stuffs about BTS, RM changed it to “happy” stuffs so his members wouldn’t sad. He also had suicidal thought, anxiety, severe depressions, loneliness, etc. Like Yoongi who made song “The Last”, RM also made song about his depression like “Suicide” and “Adrift”. And many more struggled he had gone through in the past. He deserves what he get now. I hope he is healthy and happy.
J Hope became a victim of cyber bullying. Antis and haters called him ugly, untalented, and wanted him to be kicked out from BTS. In the past, his dad opposed his dream to become a dancer. He was always compared to his older sister who is a doctor and studied abroad for her Master and Doctoral program, until he grown hate toward his sister (he said himself in a radio interview in the past). The only one who support him at that time was his mother (and of course, until now). She supported J Hope financially which was resulted in debt. But, right now, his dad is really supportive and J Hope himself show a big support to his sister.
Jimin, got bullied because of his muscular body and his baby fat. He was called “fat” and “pig”. He starved himself, only eat once in ten days and drink water. He also felt insecure about his voice. After he lose his weight, “fans” (i don’t know they were ARMYs or imposters), called him “mochi”. They called him like that because Jimin is cute but that resulted a new habit in him, pouted his cheecks to pleased ARMYs and “fans”. The other members often told Jimin to stop it. Jimin is a perfectionist and really hard on himself. He also care a lot for ARMYs so he did anything ARMYs told him. I don’t know how much this ‘mochi’ affected him, but to me, he looked like someone who lose himself at that time because of what people said to him.
Taehyung was a victim of bullying when he was in school. After become famous, he meet one of the two bullies, resolve their issue in the past, and become friends but not the other. Not because Taehyung hold grudges to him but because this bully doesn’t remorse his action in the past and even mocked Taehyung in his radio (he was/is somehow an RJ). Taehyung became secret member also gave him a lot of hard time. Fans found it surprise that BTS debut with seven members not six. They thought that Taehyung suddenly appeared as seventh member (i don’t know the details, maybe they thought Taehyung used cheat methode). His free spirited and unique personality seen as “weird” and again, he got bullied because of that.
I watched their comeback with “I Need U”, there was segment where they asked fans “what flower that match with each members” and someone posted a photo of a flower that looked like a monkey (monkey orchid / orchis simia), said it match well with Taehyung (i really shocked when watched it). Taehyung looked hurt and i’m sure the others also felt uncomfortable but they were on the show and they must being professional. I was impressed when right after that, Taehyung made a monkey – faced and the other members laughed as if it just a joke, so they didn’t ruin the show (I don’t know it was live or pre recorded show since i watched it from Youtube and it just 1 – 2 months ago).
Antis and haters bullied him, called him plagiarised a famous idol’s hair colour, antis and haters said Taehyung being rude because sang Bigbang’s “Losers” when BTS won an award even though Taehyung sang it because he like the song and admire BB, and even insulted Taehyung’s grandmother who at that time, already passed away.
Jin auditioned because of his handsome face (the world wide handsome). He must learned sing and dance from zero. He was called untalented. Same as J Hope, he was also compared to his older brother who was a role model student in his school and university (Jin said it himself at the same radio interview J Hope had).
I don’t think Jungkook had dark past. But, he was debut when 15 yo (and about 13 – 14 yo when became a trainee) so maybe, he lose his time as an odinary teenager in puberty.
In Bring the Soul docu series, he said that he don’t know how people same age like him felt in their teenager time or how teenagers felt in their puberty time, because he should work at the young age. But, he said he didn’t have problem with a lot of idol stuffs like busy schedules. He is sad and hurt when his beloved hyungs sad and hurt. And, because Jungkook has a soft and kind heart, it must be hard on him. I think, he was gone a hardest time in early 2018, when BTS facing a possibility of disbanding.
That’s all what i know until now. I didn’t include Yoongi, because a lot of people already answered it better than me. Thx.