Would the moderator please read this rule: Some adverbs such as affatto (at all), ancora (still, yet), appena (as soon as), già (already), mai (never), sempre (always) can also be placed between the auxiliary and the past participle. Therefore, the adverb sempre should be place between ho and creduto and not before the auxiliary verb.
But there are some rules, and this is one of them in Italian. As confirmed by native speakers above, adverbs always go between "avere/essere" and the participle in the present and past perfect.
Also, bear in mind that moving the adverb in English and in Italian doesn't just change the emphasis of the sentence. In some cases, it can completely change the meaning. For example, "Stunningly, the US president turned out to be corrupt," vs. "The US president turned out to be stunningly corrupt."
Again, there are no rules; only norms. GENERALLY you are right about adverb position. Citing the Italian grammar "The Italian Language Today", witten by Prof. Anna Laura Lepschy (native speaker from Milan), a Commendatore Ordine al Merito della Repubblica Italiana. In the section on adverb placement, she writes, "As will be noticed in the example (cites example) the adverb may come between the first auxiliary and the past participle. There are some adverbs for which this is the normal position: (cites five examples using già, ancora, più, sempre and mai)... " (d) The adverbial positions we have given above are the neutral, normal ones; it is of course possible to convey greater emphasis by changing the position: (cites three examples)... 'già è venuto' or 'è venuto già'. Other grammars I have referenced used phrases like 'can be' and words like 'normally' and 'usually'.
i didn't say that it just changed the emphasis, i said that emphasis is a reason to change the position. you are correct and Prof. Lepschy also cites moving positions to change the meaning. I prefer to rely on this grammar (which is written by a native speaker) rather than just a native speaker.