I think it is wonderful you think about your wife but she has the final say about whether she does/does not want to help you so being anxious when she is enjoying/helping you that she may be tired, not interested, etc. really does not help you (or her) in the long run?

My husband has a 55+ year injury to his foot so he is in near constant pain and the only thing that makes him feel better is my massaging it a certain way. Almost every evening he asks me (or I suggest) massaging it for him and I go through all sorts of feelings about it, not having time to just sit and watch TV by myself, not being in the mood for his stinky foot etc. but the bottom line for me is I know he is in constant pain and only my help can make him feel better and I love him and want him to feel better. So, I have conflicting feelings sometimes but they will not hurt me! That is my stuff and I will gladly put up with those feelings to be able to put him to sleep and have him snoring so loud I cannot hear whatever TV show I'm watching because his foot has quit hurting him (it is fun at night in bed too with his leg jerking because of the pain and waking me) for that little bit of time!

Your wife loves you! Trust that and see if you can reciprocate sometimes (backrubs when she's tired?) my husband sometimes reads aloud to me from a book whose subject we both enjoy (great "together" time and discussion!) while I rub his foot. But let your wife be in charge of telling you how she feels and what she may like/need instead of your assuming and making yourself miserable over it?