The approach I am about to describe might be unethical and perhaps even illegal. I am posting this for two main reasons: one I have no emotional attachment to your relationship; and two, because I like solving problems, the answer below is from a technical point of view: I am not really suggesting you do as I say below - the other answers that have suggested you speak with her is perhaps the best human way of solving this; I don't know: it's your call.
For this you will need to have two things:
One thing is clear: no matter what you ask her, you can never trust her answer. If you depend solely on what she tells you, you will never know whether this keylogger was installed there because she was just suspicious that you might be cheating on her, or that she is trying to build a case to run away with half of what you own. Either way a new situation has been unveiled and you will need to to be tactful; treat it like a case to be solved. The key point in your strategy is to understand her motives, and depending on the findings, take an appropriate action, even if it means divorcing her. That's your strategy: find out her real motives. Enter your tactics...
I agree that the problem is not 'technical', but 'human' (social). Now, pay attention to this: your best chance of getting to the bottom of this 'human', social problem might be via the 'technical' one. The fact that you have the upper hand, (as it has already been said here, you have the knowledge that she doesn't know that you know that the keylogger is in your machine) gives you extreme advantage in this incident: take the opportunity to use it in the following way:
- Pretend that you don't know it is there and don't use other software to scramble what you do. To the contrary, as it has already been suggested, make a model of yourself. Send e-mail to some friend (you may tell him about it or not) in a way that in the e-mail you say good things about her. Every woman I know enjoys compliments: use it to soften her. Make her read your messages that say good things about her. Why this approach? Two reasons: one, you have better room for action if she does not know that you know, and secondly, if she starts reading these words of flattery she will lower her guard a bit, giving you even more room for investigation. (I hope you have not posted this question from the infected machine.)
- Install a spyware on her cellular/mobile, such as Mobistealth for example. It is a paid service but this will give you her real time GPS location, a copy of her text messages, e-mails and phone calls. Mobiles are the best way of gathering and correlating information about a person, especially women, that carry them everywhere and used them very often to communicate their strategies with their girlfriends: they are very sociable beings, so take advantage of her social verbiage. The best time for you to do this is when she is having shower, but find out if she is going to wash her hair. Women don't tend to wash their hair every day - the rationale is simple: if she is washing her hair, you have more time to install the spyware. If her phone's OS does not support the spyware, a more drastic action is needed. You will need to get rid of her phone. For example, take her to a night club and whenever she is distracted, take her phone (destroy it and bury it or give it to a friend you trust - do not keep it at home, as she is likely to go through your stuff). Tell her how sorry you are and how this sucks, and to compensate for her loss, offer to buy her another one and give her a few options (you will try and push for the ones that support the spyware. Do some research and have valid arguments as to why she should choose one of your list. Make stuff up about phones if you want. It is unlikely she will read about the advantages of each device over another). Spy on her and find out what she's been up to. Consider spreading your infection onto her computers if you find it necessary. I bet she keeps her girlfriends up-to-date on her findings from snooping on your e-mails: this might help get to the bottom of her motives. If she is not communicating this to any of her friends, at least you get to see if she is the cheater.
If you find out that all her problem was that she was suspicious because she thought you might be interested in another woman, either because you don't treat her like you used to in the beginning, or because you have been behaving strangely, you should still keep secret that you knew about the keylogger and start acting like you were in love with her. Re-gain her trust. Remember one thing. The best seducer is not the one that can seduce many women once, but the one that can seduce the same woman over and over again.
If, on another hand, if you find out something worse, like she was wanting to divorce you to get your stuff, or that she was cheating on you, you can use her unethical behavior against herself and nullify the marriage, which depending on the laws of your country, because of her actions, she might not be eligible for half of your stuff. I don't know, I am not a lawyer.
You have asked a social question on a techy site. Pedantically speaking, the question is off-topic, as although the means of the problem is technical, the root is human. Therefore, the only reason I answered is because I am taking a cold and detached analytical solution to the problem. So treat this not as a recommendation but as an exercise of, methodical, aiming-at-being-flawless problem solving, that's all.