Does this phrase want to say "Tu parais plus sûr de toi que d’habitude"? Or "Tu parais plus sûr de toi que tu l’as jamais été"? I think this phrase is for emphasis, but I don’t understand it perfectly.
- Tu parais plus sûr de toi que jamais.
It means "more than ever".
Both "never" and "ever" translate to "jamais" in French. It can sound weird when it's use positively like "ever", and it's not very casual, but it's correct. For example you can hear (or mostly read) "A-t-on jamais vu ...", meaning a general "have we ever seen..."/"has anyone ever seen...*". It's really dated though.
But "plus ... que jamais" is still pretty common.