I just learned that close friendship with non-Muslims might not be allowed in my faith

Assalamu alaikum, everyone. I recently came across something that really shook me-apparently, in my school of thought, it’s not permissible to have best friends who aren’t Muslim, like the kind where you share your deepest secrets, love each other dearly, rely on each other for advice, and have that incredibly tight bond. Honestly, I’m feeling so down right now, not because of the ruling itself, but because I don’t know what to do next. Almost all my friends are non-Muslims, and I’d have called them my closest companions. They know everything about me, and I know everything about them… I’m afraid that if I step back from treating them as such close friends, I’ll lose them entirely. I’m also unsure about what kind of interaction crosses the line-like, how close is too close? I’ve had a Christian friend for 10 years; she’s basically like a sister to me, not literally of course, but that’s how strong our connection was. Now I’m stuck-what should I do? I don’t want to displease Allah, but I’m also terrified because I already feel lonely. I never knew about this before, and I’m scared of losing the people I care about. Any advice?