She Likes the Fantasy
It can take a long time, but it will be worth it. It took me about 2 years to get my wife Debbie to finally end up in another mans bed.
Debbie enjoyed fantasising in bed about being “naughty” with other guys and I would use a dildo or vibrator on her (not large, just realistic) and tell her how turned on I got watching another cock sliding in and out of her pussy. She loved that, but she was very wary when I used to ask her if she had seen any men she fancied or if she’d been chatted up when she’d been out without me.
Gradually though, she became a bit more open, admitting to a weakness for younger guys – she was early 30s by now, but fancied guys of about 24, 25. One night I told her that it wasn’t the idea of her having sex with someone else that would upset me (quite the opposite), but the lies and deceit involved if she did it behind my back. We even agreed various ground rules, but it was still some time before she went all the way.
My experiences are documented elsewhere on this site if you want to read them, but basicly I say take it slow. Maybe show her this site.