How do I keep loving writing when it's all become marketing?

You can get almost anything published now, and the strange grief in that is how much of writing turned into selling. You wanted to make things, and the work keeps asking you to promote them instead. That weariness is honest. The marketing is real, and it does take something from the part of you that first loved the page. But hold the two apart in your mind. The marketing is the cost of reaching readers; it is not the writing itself. The quiet hours of making something true are still there, still yours, and they do not have to be poisoned by the noise around them. Many good writers keep a clean room in their week where selling is not allowed to enter, and the craft stays sacred there. Protect one regular block of time for writing that has nothing to do with promotion, and guard it. Let that be the part that is still just yours. You cannot escape the marketing entirely, but you can refuse to let it swallow the reason you started.