> life extended - vita estesa

Honestly I don't know what Muse were trying to convey with "to live my life extended", but I guess it just means "to live a brand new, enhanced life with you" perhaps, and those final verses ("I'm busy mending broken/ Pieces of the life I've had before / Before you") seem to prove my point. Therefore, I'd go with an easy yet practical choice: "nuova vita". Some of its original meaning gets lost in translation though (you don't say), so I'll just wait and see until someone comes up with another suggestion.
It's just that translation is all about finding an equivalent people actually use, and "vita estesa" just doesn't sound right. Or, more precisely, this juxtaposition doesn't automatically work in Italian, too. "Estesa" applies to things like CDs, movies etc. "Versione estesa", pretty much like "extended version" in English.

> she could never be as good as you - non ha mai potuto essere buona come te.

I'd choose "brava" instead of "buona", although "non ha mai potuto essere brava come te" still sounds awkward to my ears. I'd rephrase it as "non sarebbe mai potuta essere alla tua altezza", which is probably what any native Italian speaker would say as far as real usage is concerned.