Tell her the one above all - relationship truth. It is that you cannot change anyone, they have to choose to change themselves. You cannot bring out all the best in someone else just by seeing it, they must make that decision to do it themselves.
Furthermore - whatever is in the relationship that is hard or difficult or not-quite-right is going to come out tenfold after marriage.
And lastly and most importantly: the best lesson I ever learned was from my Mother, she said: imagine every man you date as your husband and the father of your children and if you're not completely comfortable with that you should split. You never know what might happen if you stay.