Hi,
I haven't been in your situation so I don't know what advice to give you. I can say though that I think it was a huge mistake that you didn't talk with your daughter right after you discovered that she overheard your conversation. Possibly she wouldn't have turned to the internet for answers and information had you made her feel you were accessable and open to talking about this.
It sounds like the sites she went to were Anti-adoption sites.
Anyway there are many people here who can be of more help to you than I can. I wish you all well and hope everything works out without too much damage.
Judy