iPhone 8 storage shows wrong value on iPhone and on iTunes, so I can't restore my iPhone from an iTunes backup

My 64 GB iPhone 8's storage (from settings>general>iPhone storage) shows that I've used 55.1 GB whereas when I add up the breakdown storage (i.e. the list of all my apps, documents and system storage), it shows that I've used about 24 GB. Also, the bar at the top is COMPLETELY red (suggesting I've filled the storage with apps).

Also, these values are different from what my iTunes shows after I made a backup (iTunes shows 67 GB for documents and data, which is above the storage for my hardware: 64 GB; & 3 GB for apps). I cannot restore my iPhone from my backup (because it says that my iPhone does not have enough space). My actual backup file (from %appdata% > Roaming > Apple Computer > MobileSync > Backup) is 12 GB large.

I contacted Apple Support and they recommended that I free up space on my computer's hard drive to 70 GB, because he believed that the backup was simply not succeeding. I did this, and then backed up again, but the backup file STILL remained at 12 GB.

Additionally, I've reset my phone and computer numerous times, which did not fix the issue. I have tried installing apps/taking videos to reach the 64 GB limit, but it did not allow me to continue installing past 64 GB (which leads me to believe that the error is not simply a visual bug, and that the phone truly believes I have used 55.1 GB of storage.)

I would perform a factory reset, but because I cannot restore my iPhone from my iTunes backup (because I got an error that my iPhone didn't have enough space) I am worried that after factory resetting, I will lose all my data.

Smaller details: I have deleted safari caches, checked that iTunes, and my iPhone are updated. I am using a Windows 7 PC, which potentially explains the weird backup, but it would not explain why my iPhone is showing faulty values.