I'm so confused on this issue, and I think it's because as a reader, I think I really like backstory - assuming I am correctly identifying it.
Here is a story I made up five minutes ago which is an analog (in structure only) to my MS story, just so I don't get bogged down in my own beloved MS details:
Prologue: a nun is in the ceremony where she officially becomes a nun (I don't even know the name of this, not ordained, that's a priest, but whatever). During the ceremony, the police bust in and take her away in handcuffs.
OK, as a reader, I'm hooked.
First chapter. Same woman but flash back to some period not too long ago in the past. She's a prostitute staring at the cracks in the ceiling while a john is finishing up. OK, now I'm even more interested. How can this be the same person? As she lies there, she thinks back to her childhood when she first wanted to be a nun [backstory]. Not for too long, maybe a page. Back to the present, the john is done. Sad present-time details are in sharp contrast to what we just read about the childhood. John pays up, end of chapter.
Second chapter and beyond. Stuff happens to our prostitute, but while that stuff happens we also learn about how she actually came to be a prostitute from that girl that wanted to be a nun. Gets pregnant, tossed out of the house, those kinds of details. Some of that might be communicated in dialogue she is having in the present time, but some might be remembering back to getting tossed out of the house, and a scene describing that [backstory]. But it doesn't take over, it's not too long. Mostly the movement is from being a prostitute in present time to becoming a nun - catching up to the arrest, which let's say we are all caught up to the time of the arrest by the middle of the book. Then the rest of the book is her imprisonment/trial/love affair with the warden/whatever.
Assume that the facts of the story are set in stone, that the story in chronological sequence is: girl wants to be nun, girl becomes teenager and gets pg, pg teenager becomes prostitute, prostitute becomes nun, nun is arrested, nun is on trial etc.
In the way that I have set up the chapters, tell me why the backstory is bad. And tell me how you would structure this story if you don't agree with the structure.