I'm just putting this out there, and people will disagree with me, but IMO your binder should NEVER EVER EVER be painful. Did I mention never ever? If it hurts, it's too tight. I know people with lots of dysphoria can feel that they need to go as tight as they can stand but it's just so detrimental to your body on a whole. I would suggest making changes in attire if you find your chest isn't flat enough for you. A loose t-shirt, a loose hoodie, an extra layer, all these things will make you look flatter without making you unable to defend yourself in a bad situation nor will you be putting your body at risk. Pain means somethings wrong.
Just my two cents.