How to respond when she says 'I love you' first

She has decided to be first. That's not small. It means she's past the point of self-protection and decided the risk of saying it was worth the risk of rejection. She's also giving you information. She's in it, she wants you to know the depth, and she's waiting to see if you're in it too.

Pay attention to the context. Was it casual, in a text? Was it in person after something vulnerable? Was it out of nowhere? The setting doesn't change what she said, but it shapes what she needs next. If she said it face-to-face and you're texting back hours later, she's already reading into the delay.

First: answer the question, clearly. Do you feel the same way, or are you somewhere else? Not somewhere close, not almost there. Where are you now. If you love her, say it or say something that means it. If you're not there yet, tell her that and tell her what you do feel. Vague is worse than bad news.

Second: show her that you understand what she just did. Acknowledge the risk she took. Don't make it weird, don't oversell it, but let her know you're not taking it as a casual thing. A reply that's direct and a little tender beats one that's trying to be smooth.

She sent that into the void and now she's waiting. Every hour you take to reply, her mind is filling the gap with worst-case versions of why you haven't answered. If you need time to think about your actual feelings, that's fair. But it doesn't take hours. Even "I need to talk to you about this in person" sent within a couple hours is better than radio silence followed by a text at 11pm.

If you're unsure about your feelings, say that. Don't pretend you need to think when what you actually mean is you're scared. If she doesn't hear back and reaches out a second time, that's a sign she's spiraling. Stop that by answering the first time. If you still don't reply after she follows up, you're telling her something, and it's not kind.