Why can't i move on from my best friend's new relationship?

i’m completely desperate and honestly don’t know what to do anymore. i just need to get this off my chest because i feel like i’m reaching my limit.
i’m gay and about a year ago i met this guy at work. he was incredibly kind and caring toward me and over time i developed really strong feelings for him. Looking back there were so many things he did that made me wonder if there was something more between us.
he was always checking in on me and asking what i was doing. whenever he went back to his hometown he’d send me pictures of his journey like when he got on the bus or when he arrived even if it was 4 AM and i was asleep.
he’d constantly buy me food little keychains and random gifts. when i went to a concert he couldn’t attend because he had to work overtime he even sent me videos of himself singing along from work.
maybe i was reading too much into everything but honestly i kept wondering if this was really normal “just friends” behavior.
he left our company around may last year after working there for about eight months. i cried a lot when he left and so did he. we stayed in touch but my feelings only got stronger.
about two months after he left we met up and i finally asked him “i heard you’re bi?” he denied it and told me he had only ever dated girls. i was devastated but i swallowed my feelings. i didn’t confess because he was such a good friend and i was terrified of ruining what we had. eventually i convinced myself i was moving on.
until last week.
we had plans to go to a concert together and i made sure he had a seat right next to me. i originally planned to travel there alone because the venue was far away but he insisted on driving me because he wanted the whole experience to feel “complete.” the venue was also close to his place.
we were both worried about finding a ride home after the concert because of the crowds so i agreed to go to his house first. from there he drove me to the concert on his motorbike while another male friend of his followed separately.
the concert was amazing. afterwards we went back to his place so i could book a ride home.
then on the way back he dropped a bomb on me.
he told me he trusted me enough to finally share a secret. he and that “male friend” have actually been dating since january.
i was completely shocked. i somehow managed to just say “wow...” and act normal but inside i completely shattered.
i’m the only friend who knows about their relationship and now all the feelings i spent so long trying to bury have come rushing back. i haven’t been able to stop crying since that night.
deep down i know i have to accept reality. he’s in a relationship and i don’t want to disrespect that. but i have all these feelings bottled up inside and i genuinely don’t know what to do with them.
part of me wants to confess everything just so i can finally stop carrying it but i’m scared that would ruin our friendship and would be unfair to his boyfriend.
at the same time i’m carrying two huge secrets now his relationship and my feelings for him and i feel like i’m about to explode.