Sometimes I like to wear bras a size smaller than I should wear because I'm in denial about my actual size.
That sounds uncomfortable. Do you not know your size or do you think the size is wrong?
I should wear a D not a C cup
Wearing the wrong bra size can cause back pain issues along with pressure bruises under your arms. Make sure to be fitted by measuring your size while standing up.
It's fine I know what size I am.
Even though you know your size, shouldn't you appropriately wear it so you can prevent any back pain and chafing? How about starting by wearing larger sweaters to hide your looks until you're comfortable with a proper bra!