I don't know if there is any one solution to your problem. Maybe counseling would work. Maybe someone else telling her that what she is doing is wrong will work.
If I were you, I would definitely drive home the message that she is sinning and quote I Corinthians 7.
Physically, are you making yourself as attractive as you can be for your age? Is there anything you can do in terms of physical fitness to turn her on? Are you strong? Can you carry her in your arms?
I can't say any of this will work in a pure sexless marriage, but you can experiment with it a little and see. If you've always tried a kind, gentle, 'nice guy' approach to sex, it might be worth it to try a different technique.
You can also try a kind of hard line approach and tell her that her defrauding you of sex is wrong, and it is going to stop right now. She is going to start having sex with you. You could be really firm and unyielding about it. The weird thing about it is if that actually turns her on on some level, though on another level she may not like it. She may just accept what you say if you are unyielding about it. "Oh, I can't get away with ignoring my husbands needs anymore. Okay." Throwing in a 'you are still young and sexy and I am going to enjoy you" might work. You don't force her, of course, but you can talk to her in an insistent manner about it, and throw in the attitude of 'that's just how it's going to be'.
You can also pursue her sexually and not be deterred. Try to seduce her. One thing to do is warm her up, like really long kisses-- dip her like you are dancing the tango and do it. Show her a lot of affection throughout the day, but crank it up to the type of physical affection you would only be socially allowed to show your wife (grabs and squeezes.) Start with less sexual stuff and work your way up throughout the day. Make some comments about what you are going to do to her that night. If she says 'no' turn it into a joke, but still pursuing her. "You know you'll like it" might buy you some mileage. You can be playful about it.
If she really turns you down hard, act like it doesn't bother you, and start up the flirting an hour later. If she says 'no' and you argue with her, you are telling her she has the power to stop sex for that night with a 'no' hours before bedtime. If you argue 'but honey, we haven't had sex in a long time' you are telling her that her 'no' had the power to stop sex many hours later in the day. If she says 'no' drop the matter for the time being. She can change her mind. Bring bring the matter up again (not necessarily through discussing sex, but through action-- flirting, caressing.). She may be in a different frame of mind in a few hours after she said 'no' after you've flirted with her and shown her more physical affection. A big argument about sex a few hours before bedtime could result in no sex at bedtime. Dropping the matter if she starts to resist or deflecting it with a flirtatious joke may keep the matter open later in the day.
When bedtime comes, you can also try candles, laying there taking up the whole bed ready dressed in a way that it is obvious what you expect her to do.
The idea is not to force her into anything. Part of what you may experiment with is whether or not the idea of the insistent lover who must have her actually turns her on. It may be what she wants but didn't realize it before, especially if you've been the type to approach her in a softer manner. What I mean by a softer manner is trying to arouse her interests with flowers, foot rubs, running errands. If being the kind of lover who goes to her window and sings love songs doesn't work, maybe if you became more of a cave man lover who picks her up and carries her into the woods. Maybe she'd be more into that kind of guy. You don't force anything on her physically of course. But you can let her know that sex-starving you isn't going to happen and you insist it not happen, pursue her, and be the man who must have her. If I were in your shoes, and pursuing my wife aggressively got me some less than enthusiastic 'duty sex', I'd take it. If she gets back into duty sex mode, she may start to enjoy herself.
It's funny how acting a different way can make the night turn out differently. The husband who asks his wife meekly, "Honey, would you like to fool around?" might hear, "no." But if he comes home from work, bursts into the house, goes over to his wife, picks her up in his arms kissing her and carries her off the to bedroom with a lot of kissing and passion, things might turn out differently. Might.
If she gets angry or rejects you in a really firm manner, you can try making a flirtatious joke, dropping the matter for a while, or dropping it for the night and acting like it doesn't bother you. Don't get angry, pouty, and definitely not whiny if she won't have sex with you.
It's also interesting that sometimes we people say we want one thing, but really want another. I think us men may see this more with women than with ourselves. A woman might say she wants a husband who does more chores and that she would have sex with her husband if he did more chores. But he washed more dishes and mops more floors, and she has less sex with him. There was actually a study that showed that couples that had more gender role distinction for housework had more sex. So a wife says she would have more sex if her husband did the dishes, gave her flowers, rubbed her feet, and was so nice gentle and romantic. She thinks that's true. But what she really wants is a masculine tough guy, the rock drummer type, or the biker on the Harley type, or some kind of Roman soldier with a sword and big muscles. Or she just wants a man who acts tough and masculine, or insistent in his pursuit, and when her husband starts acting differently, it turns her on, even though intellectually she things she wants something else. I don't know what your wife wants, but it's worth experimenting with.
It also helps to think through the ethics of whatever approach you take. Is it selfish for you to want to 'seduce' your own wife, maybe even put a little healthy pressure on her to get her to do her 'duties'? I suppose it could be done with a selfish attitude. As a man with unmet sexual needs, it may feel selfish because you are trying to get her to meet your needs, and she's not met them in so long it may seem unusual and you may feel undeserving.
As a Christian man you should rule your house well. As head of your wife, you should desire your wife's sanctification. As a fellow believer who loves her, you want her to do what is right before the Lord. Her sex-starving you is bad for her relationship with the Lord. It's a sin. If you can persuade her to stop sinning (without sinning yourself) that's a good thing. It may be hard to separate out your motivations since what you are wanting from her is something your body desires, and so that may feel fleshly. It also helps to keep in mind that Proverbs 5 commands a man to always be enraptured with his wife's love. You wanting to have frequent sex with your wife is a good, noble desire. It's what God gave you as a man to 'prevent fornication.' There is even a mystery of Christ in 'two shall be one flesh.' So it is good to study the word and think through whether it is right for you to pursue your wife and how.
Also pray about it. Some of the things I've suggested won't work on every woman. If I were you, I'd really pray about it, try to figure out things in the past that turned my wife on and start experimenting with whether being a more persistent lover got any response. Part of it may be giving her what she wants. Part of it may be getting her to realize she's not going to get away with cheating you out of a reasonable, normal healthy sex life like the Bible teaches couples to have, and that she doesn't have the right to decide all on her own if you two are ever going to have sex again. She needs to take this aspect of marriage seriously and act like she is your wife.
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