A boot that does not fit is usually not the wrong size. It is one of these.

The last is the wrong shape for your foot

A high instep in a low-instep last will feel tight across the top no matter what size you take.

Going up a size makes the boot longer, not taller. That is why it does not help.

The calf is too tight

The foot fits, the shaft will not pass. Nothing about the size fixes this.

Measure the widest part of the calf and specify the shaft circumference.

It was sized down because of heel slip

Slight heel lift in a new boot is correct and disappears with wear.

A boot sized down to eliminate it will crush the toes once the leather moulds.

The heel is wrong for what you do

A tall riding heel is there to stop the boot sliding through a stirrup. On foot all day it is punishing.

If you do not ride, a walking heel is not a downgrade — it is the right tool.