“John, we were always such a great team. You and me. Even if you don’t agree with my plans it’s still me John” - Cortana The Breaking Halo 5
“John is leaving with me, when I wake him in ten thousand years, when he experiences the permanent peace I have ensured he will see that I was right.” - Cortana Guardians Halo 5
“You’ll get the hang of it.” - Cortana Reunion Halo 5 advising you on using Phaetons
I am really not sure she forgot about being a team.
Cortana spends all of Halo 5 either trying to convince the Chief to join her and failing that trap him in a cryptum until he changes his mind. Wanting to be a team is not the main issue here. If anything, she really wants that to happen.
Sure I get that they’re talking about her prioritising the plan over her bond with the Chief. The whole conflict with the Warden in Halo 5 is her not wanting to make a choice between the two when clearly she can’t have both. But still, to say she forgot about being a team is a bit off the mark. 