In this particular case, both "of" and "about" are correct, you can choose either. "About" sounds more natural and casual, but both are definitely okay. There are many other circumstances where these two words' meanings are quite different, so be careful!

The second one would be correct however, the best way to struct that sentence is to change "sure" to "certain." --> I have never been more certain about anything in my entire life. But either one would be fine. :)

Both sound fine :) Maybe the second one sounds a LITTLE tiny bit better, but both are really okay.