Its one of the hardest things to say to another person, its ok to say i love such and such but to say i love you is not something IMO that is freely flown around.The girl maybe will say it but then again she may not i think you should learn a little more about body language.
To say i love you to someone is IMO a gift you only say it to that special someone you have bonded with no matter if it's gf/grandparent/etc its a sign of respect but in a greater meaning.Not something that actually means nothing to you.
A mistake is always forgivable, rarely excusable and always unacceptable.
Robert Fripp