found this, although I don't put too much stock in it
4 - Are both spouses guilty of a mortal sin if one chooses to use a contraceptive against the others will?
No. If a man decides to have a vasectomy against his wife’s will, she may still have sex with him and not be guilty. Likewise, if a woman is taking birth control pills against a husband’s wished, he may still have sex with her and remain guilt free.
Pius XI teaches the same thing in Casti Connubii
"59. Holy Church knows well that not infrequently one of the parties is sinned against rather than sinning, when for a grave cause he or she reluctantly allows the perversion of the right order. In such a case, there is no sin, provided that, mindful of the law of charity, he or she does not neglect to seek to dissuade and to deter the partner from sin." http://www.vatican.va/holy_father/pius_xi/encyclicals/documents/hf_p-xi_enc_31121930_casti-connubii_en.html
Notice an effort to correct the spouse must be made, even if it doesn't succeed. If there is no practical change, keep praying that she has a change of heart. There's nothing more you can do. But there's no obligation to abstain.
If your conscience still troubles you, talk to a priest for reassurance (don't go near a Novus Ordo priest, and find an absolutely traditional one who teaches that NFP can only be used for a grave reason). He will tell you the same as Pius XI, but it often helps to have a priest's personal assurance that you are doing nothing wrong, and he may pray for your wife as well. Have a Mass said for your intention (that your wife stops using this method).