I don't think you're out of chances, if that helps. I also don't think you're helping yourself when she tells you that she needs to "feel a spark" for you or she'll be headed for divorce, and you reply that the spark isn't important as long as you're comfortable as companions. If she tells you what she needs in the marriage, and you tell her it's not important, you're telling her that you shouldn't be married. Think about the most important thing in the marriage--the one part of your relationship with your wife that you couldn't do without. Now imagine her explaining that it's not really important because there are other good things in the marriage. Would that convince you?
She feels low desire for you and she feels angry at herself over feeling low desire at the same time. It doesn't have to make sense to you right now as long as you understand that it's likely true; I still don't completely get how it works.
You're going to have to think hard about why you want to stay married to this woman. You seem to be saying that you missed the sexual "spark" when you first noticed it fading, but have now adapted. So why be married? Why not just be buddies, especially since you don't apparently have kids? I don't ask that because I buy it, but because she may be thinking the same things. Are you sure you can live without that "spark" between you? It seems like maybe she can't.
Have you read the Michele Weiner-Davis books this forum is about, like Divorcebusting or The Sex-Starved Marriage? (I would start with The Sex-Starved Marriage book if I were you.) Other books often recommended here that have helped people in similar fixes are No More Mr. Nice Guy and The Passionate Marriage. (Basically I'm only recommending books I've read myself, so if other people recommend other things, that doesn't necessarily mean I disagree with them.)
The Sex-Starved Marriage: Reading this is what helped me break through to my wife. She had low/no desire for years, and I'd been pressuring her and fighting her for years without ever making much of a dent. SSM showed me that other people had my problems, that things I only dimly understood had names, and that there were ways to get what I needed and give her what she needed if I was willing to take the risks and do the work.
No More Mr. Nice Guy: This book by Dr. Robert Glover is maybe the least useful of the three, and it won't help unless you have traits of what Glover calls "Mr. Nice Guy." But from your post, it sounds like you could benefit from it. "Nice Guys" in Glover's parlance are men who believe that aggression and arrogance are so bad that they have to "be nice" all the time--only, "being nice" here isn't really nice. It's a sort of servile, bland, cringing attitude that you have to avoid "being a problem" as much as possible, and it means you don't stand up for your own needs, and it can often mean that you have a hard time being genuinely nice because you resent all the things you've done or given up without getting something in return ("being nice" as part of an arrangement to get something in return--usually without telling the other person that you expect to be repaid--is another trait of Nice Guys.) Nice Guys think that if they just "be supportive" enough and give up enough (like sex with your wife) then she'll be obligated to stay with you and love you. But it doesn't work like that, and it's so insidious that it's hard even to recognize this kind of thinking in yourself.
Passionate Marriage by Schnarch: I'm only getting around to reading this right now myself, and I put it last mostly because it's the longest, most involved of the three. But I do think there are important ideas in here. Schnarch emphasizes "differentiation," the ability to soothe yourself when you need it and sustain yourself close to your spouse. The opposite is "fusion," wherein you need your spouse because your self is tied to their self--you get validated by reflection from them. This is draining for both of you, and leads to people who are "too much in love" (like the husband who beats his wife, and the wife who can't leave him even so) or people who say "I love you, but I'm not in love with you" (people who care about each other because they're emotionally fused, but don't feel like they're in love because they're drained from the effort of completely supporting each other and not supporting themselves.)
She may not want to join you in this at first, and that's OK. Do what you can. Maybe start by thinking about the question above: Why do you want to stay married? Why is that better than divorcing and being friends?
