Really you have to do what you think is best. The question for you is do your vows matter at this point. Clearly she ignores the "love, honor and cherish" part. But you're still hooked on those, maybe because of her illness, maybe just because you're too nice a guy, maybe you just don't want to break the same part of the vows she is. How I was raised I was taught to end the contract before breaking the contract.
I would definitely recommend a shrink, having someone to dump your problems on is good, and it's better to do that with a shrink than a mistress. One problem you face that that adultery can be proven in court, being a bitch can't, if you cheat you're risking getting seriously raped in a divorce. Depending on your financial status you might want to contemplate just what it would take to provide enough for her to not feel guilty about leaving her, could you pay for a live in nurse for her and leave with a clean conscience. Also just how survivable is the disease. I know it sound cold but if her life expectancy doesn't make it out of this year think of it like purgatory and tough it out. This is especially true if she has cute friends (funerals are good places to hook up, sad but true).
In the end you're the one that has to look at yourself in the mirror every day. What we say doesn't matter. If your friends (or FRiends) won't accept your decision get new friends.
SHE has to honor the love, honor and OBEY part - HE has to honor the love, honor and cherish part. It's an important distinction, an acknowledgement of the different roles of men and women - and of the same expectation to be honorable and loving. Cherish is not the same thing as love.