Killmonger never lost a fight one-on-one against BP. Only one not to. He has BP's number.
Doom beat him when he had the power of the beyonders, despite BP having the infinity gauntlets.
A possessed Iron fist hit him in the head one too many times, giving BP a seemingly incurable brain aneurysm.
BP biggest L's though, are often internal and often involves his personal life.
He lost his marriage with Storm, after Namor's attack on Wakanda.
His uncle got murdered by Doom.
Two of his closest advisors, W'Kabi and Zuri, got killed by Morlun.
The Dora Milaje turned on him when he harbored Namor in the Necropolis during the Wakandan-Atlantian war.
His relationship with Shuri, who was the Queen of Wakanda at the time, was severely strained when she found about Namor being the Necroplis during the Wakandan-Atlantian war. She banned T'Challa from entering Wakanda due to him harboring Namor during the war.
When T'Challa chose not to blow up an inhabited planet in order to save his own Earth, T'Chaka and the previous Black Panthers disowned him.
Thanos and his Black Order destroyed Wakanda, committing genocide on Wakandans.
After a last ditch effort to stop Thanos's group failed, Shuri helps T'Challa escape what's left of Wakanda and decides not to go with him (they squashed their beef and made up). She gets killed by Proxima Midnight, one of Thanos' Black order members. T'Challa becomes the only Wakanda left.
T'Challa thought he killed Namor, but Namor managed to surviving his attack.
BP went through it for a good 3-4 years. It was a crazy time back then.