Shepuxsucks to be you lol
seriously who the hell buys boots by trying only ONE of them on and without the aid of a boot fitter?
on the bright side, there is no real difference in the actual liners- they are only heat lasted to be slightly slightly narrower. no thickness differences between whole and half size. Once you get them properly heat molded, they will be identical and fit properly. The boot fitter there should have done this for you anyway.
plus, i doubt your feet are the same width left to right. This is also why you should have had a boot fitter look at you. There is a high chance the pressure you are feeling is due to differences in foot width/pronation between left and right.
take them back and ask to see their boot fitter and have it actually fitted to your foot with good footbeds.