Coop - maybe you feel this way because it is naturally hard to take care of someone you love, and especially to see someone that you know is strong and be weak and needy. (Not that Lisa is being weak and needy in a bad way - just that after surgery, everyone is weak and needy.) Maybe its just hard for you to see someone that you think of as "strong" in that way? Maybe it is a little scary when you are then put into the position of the "strong one".
I don't know, just thinking out loud and offering a "Maybe".
It sounds like you are doing a terrific job and Lisa is a lucky girl (and you a lucky guy for having her). Just try to cut yourself some slack and don't be "superman". I think my husband tries to be superman - and I always think of him when I hear that song Five For Fighting Superman song. Great Lyrics - could apply to a lot of survivors.http://www.wowlyrics.com/read.php?wow=1604560
Take care - you're doing great. Just take some deep breaths. Maybe very confused is onto something with that Yoga stuff...