I will suggest writing something like this "Its okay I understand but I thought we can ...." and tell her something interesting that both of you did on your first date. The main idea is to bring back the emotions she felt on her first date. You can also send her a pic of your first date (if you have). I don't think chemistry is her real concern and she won't tell you what the real problem is if you ask her directly. Alex has already made a video on this topic. You should check that out too.
