How to reply to a girl who says "I can't stop thinking about you"?

The part after the statement matters most. When she adds "ignore me if that's weird" or "tell me I'm being dramatic," she is asking whether she is safe to have said it. Most of the work of the reply is answering that question, not the feeling itself. If you treat the message as a test, it becomes one.

Also notice how much she has told you about the date. If she says it after a good evening, the statement is mostly about the evening. If she says it with no real context, like in the middle of a quiet week, she may be lonely or testing how you react. Both are fine, but they call for slightly different answers. You don't need to figure it all out before replying. You just need to read the question under the question.

A reply that fits this situation has three parts. It receives what she said, it gives her a concrete detail that shows you were thinking about her too, and it sets a comfortable pace. The receiving part can be as simple as saying it's nice to hear. The detail should be something from the date, not a generic compliment. The pace part is the tricky bit. You want to be a shade calmer than her, so she has something steady to lean on.

What to leave out: anything that claims a feeling you haven't had time to form, any joke that makes the statement sound silly, and any question that asks her to defend or define it. Keep the reply short enough that she has room to answer. The best version reads like something you would say to her face if she had just told you this across a table, with the same warmth but without the jolt of a big declaration.

Send the reply within a few hours if you can. Late at night is fine, but if she sent it in the evening and you go quiet until the next afternoon, she will have spent a long time wondering. If she says she was just being silly, don't push back hard. Lightly agree that it's a fun thing to say and move to a plan or a question about her day. That keeps the door open without making her feel she has to justify the message.

If she goes quiet after you reply, give it a day or two before you follow up, and make the follow-up about something other than the statement, like a plan for the weekend. If she doesn't respond to two attempts across a week or so, drop it. Silence after a vulnerable message usually means she is embarrassed, not that she is waiting for you to clarify. Let her come back on her own time, and don't chase her with more feelings-talk.