It completely slipped my mind. Honestly.老实说我真的忘记这事了
slip one's mind忘记
It slipped my mind that I was visiting him today.　
我忘了今天要访问他的事。
I'm sorry I forgot your birthday; it completely slipped my mind.　
我很抱歉忘了你的生日，我完全记不得了。

with everything that's going on,I totally spaced about Emily's party.发生这一切之后我完全忘了艾米丽的派对
go on发生
Are you alive to what is going on?
你注意到正在发生的事吗?
The new play will go on the boards next month.
这出新戏将在下个月上演