A girl with a very good relationship sent a message saying that she loved me.

It is normal for a girl to say "I love you" if she has a good relationship. If she tells you seriously and then sends you a lot of words, she may like you, so you don't have to worry. I have had such an experience, you don't have to tell her clearly that it is impossible or how, because it really hurts, unless she wants to talk to you about it formally. You can usually imply that you like your penis or something. Don't be too obvious.