“I didn't mean to do this”, could this be turned into assertive sentence?
"I didn't mean to do this", could it be used in assertive way, "I mean to do this"

Question

Answer

The assertive opposite of "I didn't mean to do this",  is "I meant to do this".

Answer

Yes.

"I meant it!"

is a perfectly valid and very useful sentence.
for emphasis "Damned right, I meant it."  And "I did it on purpose" are also ways of saying something similar.

Answer

I meant intentionally to do this.