It isn't fair, it is awful even. I can barely imagine how shitty I would feel in your stead. There isn't anything I can tell you that would make it easier and I really appreciate that you took the time to vent- we NEED to do so sometimes.
I will say that if things needed to get better for her, then getting the best chosen medical options from a professional is likely the best thing for her. Try to focus on the positive change being worked for, the help and decision to fight for an improved life and family journey in the future. This very hard struggle 'to get better' is infinitely more valuable than denial, pretending things aren't 'that' bad yet or just generally NOT making the hard decisions and follow through of real action. A person can only do their best, in my experience as hard as it can be to face our problems- the solutions are oft even more difficult to face. I do believe there is truly something honorable when we do our best and face those hard solutions. This is yet another bad week in the 15 years you've been with her, yet another chance for you to grow, to widen your perspective and your resolve, to remind yourself what you are fighting for, and to continue to do as you've always done for 15 years. For fifteen years you have proven that you are NOT WEAK! You've time-crafted knowledge for yourself and your wife that you are unbreakable, unyielding, and unwilling to let your wife face her problems or the hardest solutions alone. You were right in another post that people might call that honorable, but I say that just is just what it is! That is you.
There is nothing weak about crying or caring. Crying like a baby can be cathartic and the only thing that keeps us from breaking. Caring the way we care- that's the only way I know how or will choose to live my life. This must be exceptionally hard, but she has needed to lean on you in worse, your parents are there to help with your daughter (and there is nothing wrong in accepting said help), and your wife is in a very hard place right now knowing for sure I suspect- that her husband loves her, is understanding, is forgiving, and has never let the struggle scare him away, but always had her back when it counts. I've been in an adult stress unit as a teen- I've also been in prison a bit later on drug charges- in both, with everything in the world stripped away (including your freedom) you reflect as I imagine one would on their deathbed. I did anyway, and all I could think about were the people I loved and how significant those relationships. I wasn't thinking about the problems I had, but the solutions, I wasn't regretting facing trials, but happy to have had the chance to celebrate victories.. I was ever grateful for those VERY FEW who loved me, who never gave up on me, and who respected and 'knew' me aside from bad behavior or temporary, distorted identity. This is a chance for tomorrow. Tomorrow you are going to continue being the man that is unbreakable- to be unbreakable does not mean you feel no pain, or even that you take no permanent damage.. it just means you keep going and keep maintaining and even rebuilding yourself. You know you can do that. If you were like many here or elsewhere, you would have already left your wife. You decided to stay and make the most of it- you are halfway through hell and I suggest you keep moving forward. It looks dim right now, but it isn't. You CAN outlast your problems. Don't let the past be a forecast of your future. Your wife has a little girl in her, not unlike your daughter, who desperately needs you to be strong- and you ARE strong. YOU DID NOT FAIL HER! She isn't there because you couldn't fix her! She is NOT where you get your worth of your approval. YOU ARE. When you can truly hold onto that, as hard as this is it can be with a focus moved from pain and to the positive. She needs you to be the leader in your home- she cannot lead her own life right now without your help. I know this reeks of being a shallow pep talk, but it isn't, it is just some truth I wished to point out to you as we get so lost in our own stories sometimes that we miss them.