The book is definitely Scatterlings by <PERSON>. I've tried to answer all of your recollections with quotes from the book - you've got a fantastic memory! Hope this helps.
The book starts with a girl waking up in a vehicle crash.
The girl’s name is <PERSON>. She wakes up in the wreck of what she thinks is an ambulance but is actually a “flier”
She was in the back of a van surrounded by blankets, pieces of broken glass and unfamiliar implements. Through a dark tinted window under her, running the length of the cabin she could see foliage pressed against the glass. A matching window above showed the sky. Clearly she had been in an accident and the van had overturned.
Ambulance, she thought dazedly. I was in the back of an ambulance so there must have been something wrong with me before the accident.
Then later on…
“Pity you didn’t come sooner,” <PERSON> said absently. “One of <PERSON>’s old traps netted a Citizen gods’ flier.”
She explores the planet - she walks through a city that has been overtaken by plants/trees.
The city itself was a ruin…trees, once confined to circular grills had grown to a monstrous size, their serpentine roots twisting and writhing in all directions, cracking open what remained of sidewalks.
However she has flashbacks of the city with thousands of inhabitants, which obviously happened hundreds years ago.
Around her, the city seemed to reshape and reform before her eyes, assuming a ghostly familiarity…but her memory was of a vigorous, busy, populated city, not an ancient ruin.
She meets some tribe people and they have telepathic powers.
Again his lips did not move and <PERSON> realised, amazed, that he was reproaching her telepathically.
The tribe people are terrified of another humanoid species that lives on the planet - they are technically advanced but don't have telepathic powers.
The tribespeople are not really terrified – they just think of them as gods who have great powers. Thus the name “Citizen gods”.
“The Lord warden of our clan said we had no right to demand mindbond of a god; that a god is truth,” said another.
She accompanies the tribe to a large meeting in a valley and discovers that she also has telepathic powers, though she struggles to control them at first.
She discovers that she has a telepathic block like the ‘Citizen Gods’ at least at that point in the book. 322
It’s only much closer to the end that she realises she has telepathic powers.
“Why do you refuse to Accept me? How do you close your mind like the Citizen gods?” she heard him ask.
The tribes people have some weird pseudo-religion where they send off tribesmen to the other species as a sacrifice and they are returned physically untouched but with there brains completely and irreversibly wiped of personality/memory.
Yep, but it’s more a combination of law and religion. And the sacrifices return pretty much like drooling idiots.
“The clans tithe children to the Citizen gods, just as they tithe to Conclave. This is for the good of the many. You know that. Before this, our people were hunted down and herded like cattle. Many died. It is for the best.”
And then later…
“You gave the boy up because it was the law, and because it was for the boy’s good as well…once chosen, he had the chance to become <PERSON>.”
It turns out that the Citizens actually created the religion to get the clanpeople to want to sacrifice their children.
“It was <PERSON> who came up with the idea of making the clanpeople want to come into the city. She designed a mythology which focused around the city and the Citizens, and she brought in and brainwashed a number of tribal wardens using a highly addictive drug to ensure they were obedient.”
She's really conflicted as she's not sure if she belongs with the tribes people (who she looks like and shares powers with) or with another species who she remembers in flashbacks.
Also, because not a lot of what the tribesmen do and how they think makes sense to her, and from what she hears, the Citizen gods sound more like her “kind”.
In the end she goes to seek out the other species.
Tthe tribespeople send her to the Citizen gods as part of the clan tithe. This is because a person called a “Rememberer” (a type of tribesperson who can see the future) told the clanspeople to do it. 72