Hi maybe I'm a little late but being a girl I may have some input.
First if she does not want to share something with you its the end of it and respect it. If she is as mature, she will respect you for this. Don't pester her with with something she does not want to talk about. Maybe she's not very comfortable about it. Although you have been together 17 years there's a different connection spiritually. Maybe there are some things she would rather keep. And its not always got to do with anything physical. Maybe it was a bad experience mentally.