what if it's not ADHD just a crummy marriage??

So I have been posting here for a long time and reading all the post for a long time and I am starting to wonder if DHs ADHD is not the issue, we just have a crummy marriage that has run its course.  It's been 20 years.  Maybe that's all we have in us.  I just don't know what to say or do any more.  DH comes and goes as he pleases, sometimes tells me when he is coming home, sometimes not.  Today he was supposed to be home by 4:30 and instead walked in at 7:45, but only because he had taken his car to have a recall fixed.  So how do I get mad at that??  What kind of a shrew wife is upset that he took care of the car???  And then he showered and ate and sat and read his book until he went to bed about 10:30.  Okay then.  We discussed sex at our last marriage counseling.  DH is fine with once a month.  I am not, but what can I do?  He travels Monday-Friday most weeks and when he is home on the weekends he is exhausted.  So again, what kind of shrew wife is upset when their husband is tired?  We never go out--he's tired and the last thing he wants to do after sleeping in hotel rooms and eating out is to go out again during the 2 short days he is home--I get that.  So I do all my fun things while he is gone--out to dinner, theater, opera, dancing, live story telling, lots of stuff, and then I resign myself to being home on the weekends.  Is that normal?  What do other couples do when one spouse travels?  I cannot possibly believe that every couple who has a spouse that travels does nothing with them during all the time they're home.  None of those seem to be ADHD problems.  He is on the periphery of our lives (I have two boys--ages 14 and 16).  Since he is not here, most of the managing of them falls to me.  There is a lot that goes on that he never knows.  Not on purpose, but stuff goes on while he's away, and the moment has passed by the time he gets home.  

I can't explain it--I just think maybe this has run its course.  I feel lonely and bored pretty much all the time.  I dread when he is home--I feel so much more myself when he's not here.  I am more playful with the kids, more relaxed at home.  When he is home, I am on edge, quiet, careful about what I say to him so I don't stress him out (he has suffered from depression and PTSD and has been suicidal in the past).  He tells me he can handle anything but many things I tell him, he overreacts to.  My 16 year old is dating his first real girlfriend.  They hang out at her house a lot, in the basement, where her mom doesn't check on them very often.  I don't like it, but I cannot tell another mom how to run things in her house, I can only beat it into the 16 year old's head to NOT go too far with her.  DH suggested we call her mom and ask if she minds if her daughter has sex at age 16.  Um, NO, in what universe is that the appropriate response????  How is that calmly managing things??  We sit in marriage counseling with not much to discuss--there is SOOOOOO much that doesn't match up with the way that we see the marriage, I don't even know what to say or where to start.  None of this is due to ADHD.  What is the marriage is just crummy and over??  What next??