Question

what will tomorrow become in past continuous tense​

Answer 1

This is colloquial spoken English, and without the rest of the conversation it's a bit difficult to understand.

The speaker is already dreading tomorrow because they have to give something back to someone. It sounds like they have just been presented with some additional reason to dread tomorrow, leading them to exclaim:

"like I wasn't dreading tomorrow enough having to give it back to him"