Let's assume for a moment that part of the problem in a troubled relationship where one spouse has FMS is that s/he isn't living up to that part of the contract. If this is so, then anger on the part of the well spouse is an understandable response, as is guilt and/or defensiveness on the part of the PWF. It doesn't matter that neither the anger nor the guilt/defensiveness is rationally justified. No one is to blame for feeling the way s/he does. Feelings are facts that must be dealt with, by both parties. 

What to do? If this rings any bells with you, how about a conversation on the subject, some time when interruptions are unlikely and things aren't too tense for a calm appraisal of the situation? How about an honest expression of regret that you can't do all the things you used to be able to do, an expression of hope that the day will come when you're able again, and a promise to learn everything you can about FMS to make that day come as soon as possible. Make it OK for your SO to express feelings of anger, fear, frustration, whatever, without your feeling attacked and criticized. Acknowledge that, for the time being at least, you've each lost something of value in the relationship, and promise to help each other retrieve it.