I have never been this bad would be translated like this :

Je n’ai jamais été aussi mauvais.

I have never been is translated as “Je n’ai jamais été”, as you said. This bad is translated there by aussi mauvais.

This in this kind of sentence is always translated as aussi. So, for your other sentence, I have never been this early would be translated as :

Je n’ai jamais été aussi en avance.