saying i love you doesn't mean much, in comparison to acting as if you love her. the whole action speak louder than words thingy. I am sure she realises this and you aswell, since you both get the gushy feeling
Although it may be nice to hear someone else say it, there no point in trying to get her to say it, if you feel she may be uneasy talking about relationship stuff. Each others actions will imply more than just those words, so i don't think its too important.
Although if you really want to say it, probably do it in the letter for now. Although may i ask, how long you've been together?
Normally its the other way around (guys can't talk about feelings etc), but if its still early in the relationship, you don't want to come on too strong.
Give it some time and everything should be in its right place.