Hi!
It is strange that your iPhone shows more used storage space than its capacity. It could be a display error. Have you tried restarting your iPhone and checking memory status again? To do so:
iPhone 8 and following:
Quickly press and release the Volume Up button
Quickly press and release the Volume down button
Long press the side button (power) until the Apple logo appears
iPhone 7 and 7 Plus:
Press the Volume down button and the Side (Power) button simultaneously with a long press until the Apple logo appears
iPhone 6s and following:
Long press the side (power) button and the Home button simultaneously until the Apple logo appears
If the problem persists, you might try to make a backup of your photos and restore your iPhone to see if the problem is not solved. To do this open Settings, press on Your Name, then on "iCloud", then on "Photos" and activate the option "ICloud Photo Library".
Then reset the iPhone.
I hope I helped you!