I wish I knew how good T-Ara were and discovered them sooner. Even though I wasn't that late of a fan (I got into sometime in 2013ish I don't remember what song or when in 2013/2012/2014 I became a fan though) I still feel like I missed out on T-Ara's golden years unfortunately.
Also, in a similar vein, I wish I would've been open to listening to more boy groups when I first got into K-Pop because I missed out on a lot artists that I could've gotten into if I wasn't so uncomfortable with my own sexuality. I was so afraid of doing anything "gay" even though I was already out of the closet that I missed out on Infinite, ZE:A, Teen Top, BAP, F.Cuz, MBLAQ, 2PM, U-Kiss, etc. I've gotten over it now, but I regret that that was the reason I decided not to listen to their music because I have definitely done a lot more actually gay things since then lol