Offer her some options that will give her more freedom (and yourself as well!). You know, she's just having an early mid-life crisis. You would no doubt start feeling the same way eventually too since men usually don't have them till a little later.
And she's right, it's nothing to do with you, don't take it that way. If you do, you won't be able to think rationally about a solution. Tim and Xena are doing the open marriage thing and it seems to be working for them. You could try going to a swingers dance (without actually having to have sex with anyone, LOL) and just enjoy the attention. She might love that (or hate it, you never know). Joel and I did that last month and we just met up with a couple from the dance and had a very sexually liberating evening with them both. We were like nervous teenagers getting ready for a date. We were in it together and it was very exciting for us both. Of course, we only agreed since we both felt comfortable with the other couple. I don't know if we'll do anything as freaky as that again (yikes!) but we both know that we can put things on the table and not be afraid of each other's reaction. The freedom is overwhelming.
Start being unconventional. This is YOUR marriage. Who says it has to feel like a prison? If you think you're going to lose her anyways, why not give up any jealousies or insecurities and just be her friend and both explore the world by "aiding each other" to do so. It's worth a try.
I hope I don't sound too wild or immoral to you. I'm just surprised as hell that I am happier than ever with Joel. I'm constantly amazed because I really wanted a divorce so bad.
Good luck and hang in there.
Anne