I think that was a healthy thing to do to write her that letter. I don't know if i would suggest writing her this second letter or telling her however you intend to about her not being who you thought she was.... though i suppose she should be made aware. If you do decide to tell her/write her, etc, i would make sure to be as non-confrontational as possible... you don't want her going on the defensive... or offensive for that matter... i would explain to her how you feel about her response to your first letter, and how you feel about yourself regarding your giving nature... though im sure you would know what you want to address... I wish you well.
~Julie

"Sometimes it takes us to the bottom of our misery to understand the truth..."
~Gustav Havel - existentialist