To the people of such a time, faith was more than a belief it was fact. Someone as vile as Cersei should genuinely believe in the Seven and agonise over her misdeeds sending her to hell, as many Medieval Knights felt
One should not confuse people being
religious with people being
pious.
Medieval knights, allegedly highly concerned about being sent to hell for their misdeeds, thought nothing about chasing Pope Gregory VII into Hadrian's mausoleum and keeping him under siege at the behest of excommunicated King of the Germans Henry IV
.
The idea of Cersei agonizing over her deeds in the night - it's not medieval, it's 19th century romaticism.
Your typical RL Cersei equivalents agonized over which cleric to raise as (anti-)Pope and how to get rid of the current one. Hence events like Western Schism, when Catholic Church spent half a century in a state of collapse and had to be un-borked by the Emperor Sigismund.
And it worked - mostly because Sigismund harbored no illusions about his contemporaries and their piety. So he spent a few years spending big bucks and busting balls until finally everyone agreed to stop this shameful display and restore a measure of dignity to the office of the Pope.
Also, there was a lot of luck involved. Because French court considered invading Italy and getting rid of Roman Pope by force of arms and thus legitimizing their own Pope. Fortunately for the Catholic Church, the English invaded and French became too busy being beaten and humiliated for any foreign adventures. But it is a sufficient demonstration of how RL Cersei counterparts viewed the world, religion and Holy Father in particular.
So Cersei is pretty typical in that regard. She engages in replacing a Pope and tries to play politics with the one she can't simply get rid of. With a caveat that local Pope equivalent is much less important politically, so one does not have to spend nearly as much time on the matter.
So the Faith of the Seven are nothing?
Faith of the Seven is not magical, if that's what you are implying.
Then again, so are all other faiths. The Old Gods, the Drowned God, the Red God - they do not grant power or manifest themselves in any practical way. A few of their followers that have magic, but it's a case of a wizard being religious, not religion guaranteeing D&D-style divine powers.