Knowing whether to stay or go...

I'm currently in a marriage of ten years that has been gradually going downhill for the past 8 years.  My wife and I got together when we both had young kids from previous marriages, and I feel like we did a great job of raising a stepfamily despite all of those challenges that come with being a stepfamily. Now, two of our three kids are off in college and the youngest (my stepson) is 16.  So, he hardly needs (or wants) our continual attention, but he does need care like any kid. 

One thing I have noticed is that, now that most of our kids have left home, there really isn't much of a connection or shared goals between the two of us.  Maybe there never was a connection beyond the intense courtship phase other than our kids.  Our life has been scattered and disorganized for as long as I can remember.  My wife mentioned that she had been diagnosed with ADHD as a child, but has never received treatment or expressed any interest in treatment.  Now that I have learned more about ADHD and its effects on marriage, we're a textbook case.  In addition, I've noticed that my wife has a lot of symptoms of narcissistic personality disorder and has several untreated physical ailments.  They all feed off of each other and a lot of times I feel like I am married to a black hole that eats all of my energy.  I've seen what situations like this can do to a couple because my wife's parents suffered through 50+ years of unhappy marriage due to similar conditions.  Overall, my wife is a nice person and a very accomplished researcher, but I feel like beyond that, our life is in shambles.

Honestly, I am so exhausted at this point.  I'm not the person I want to be, and I'm not living the life I want to live.  If the only problem was ADHD, I feel like I could work through treatment and rebuild a life with my wife.  But, I'm 50 and I don't think I can commit the rest of my healthiest years to address three major challenges just to get back to a point where we both are happy.  I want to leave and I often daydream about how nice life would be on my own.  I would also feel terrible for leaving my wife because she seems a bit hopeless day-to-day.  I would love any advice you could offer.