I can't believe I forgot

= How could I forget

= How come I forgot

我竟然忘了；我怎么恨高都忘唉立沙了

I can't believe I forgot what I had for dinner last night.

我竟然忘了昨天晚饭吃的什么了。

How could I forget what I had for dinner yesterday?

= How come I forgot what I had for dinner yesterday?

我航究怎么都忘了昨天晚饭吃的什么了？

（图片来源于网络）