Once you tell a girl you love her, does she subconsciously lose interest?

You are absolutely wrong (no offense lol). Every girl is different, but most of them are thrilled to hear those words (if the guy truly means it). Saying you love somebody doesn't mean your sappy either...it means you LOVE them! As long as your not clingy, and you don't overwhelm a girl with how much you say it, then there's no problem :)