I frankly grew out of Harry Potter, but it feels like needing to separate half of any author from their work is just, well, a constant. The world's most successful creatives are put in a position of significant power, becoming voices that people listen to, and commanding major brands. These voices will very often be transphobic, either overtly like J.K., or even covertly by, say, continuing to present trans rights as something that can be debated or compromised on.

Or essentially, don't think that somebody who can afford to live in a country estate has your best interests at heart no matter what. J.K. is a billionaire. She lives in a social stratum that you will never see, much less a minority group with as high percentages of unemployment and wage disparity as trans people. Folks like J.K. can get higher up the societal ladder because they rock it less, and once they're at the top, they have no reason to sympathize with those below, do they? If anything that would be against their best interests.

Alright, so that explains why your favorite author or musician or film director somehow always ends up being a racist antisemite, a queerphobe, a nonce, or whatever else they always end up being, but that doesn't explain what to do about it... It's a tough question, but I think the first thing you can do is to realize that you work for them. Because it's the proliferation of a creative work, and the innovations and commentary on it, that create most of its value, right? This is why some people say that piracy is good for business: piracy doesn't truly "lose" sales, because someone who pirates for financial or even moral reasons wouldn't have bought that work no matter what, so at least they can still contributes to the value of the product.

Or in other words, your contributions as a fan create surplus value for your de facto boss to reap. This means that separating the author from their work is effectively the same as separating the boss from their mills: you need to take what's theirs and make it ours, or put simply, you need to abolish copyright (among other things).

Buuuut you aren't exactly in a position to do that, so for now, just chew on that thought and act according to your best intentions.

Personally, I usually just turn my brain off when I play or watch or read or listen to something made by a transphobe. It's really the only way I can stay sane.