Consent issue

My boyfriend always tries to finger my butt during sex. I don’t like it at all, I’ve told him since the first time he tried it that I didn’t like it. He has yet to stop and it results in me crying and feeling like shit after sex. I HATE to tell him no and this is the only thing I’ve ever drawn a line on and for a year now he hasn’t respected that. Part of me feels guilty telling him no anymore Bc it’s obviously something important to him and I don’t want him to seek it elsewhere bc he’s done so before. I don’t know what to do :,(