So, なかったこと can mean "thing that never happened"... I see. And できませんか referred to it like "could you [make like] it never happened"...okay. I thought she said "couldn't/shouldn't I do it?"
And "喜ぶの我慢するよ”, I thought, was "I will happily patiently wait [until you say it]"
What a tricky language. @Leadrous it's called "investition". She will get ROI a little bit later.