If you buy a new shoe and it bites what do yo do?

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i bought a new shoe which was rather costly about a month ago. At the time I wore it and found that it was a little tight but i thought it will loosen up afte a few days and fit jsut right. My earlier pair was a little lose and it became more lose after a few days and i found it a little too big. now after wearing it for about a month,i find that it has not become lose and it still pinches my leg. I do not like wearing it and i cna not return it to the store too as it has laready been used for a month. What do i do to make it loose? I have to wear it for a good 6 month sor so for it to becoem loose, but by then my poor feet will be in bad shape. Is there a way to make it loose? i can't throw it either as it is quite costly.