Gosh this is a sad thread OP, I am sorry you are where you are. Do you just have 1 DC? I think what I find sad is that this is so so common at DCs leaving for Uni age. (I know youve said its been difficult for a couple of years...but the 'almost adult children' phase and potential empty nesting does prompt re-evaluations of situations).
What I think I find sad is that (Im assuming here that DH is DCs dad) is you guys have done fantastically well, nearly 20 years together and raised a succesfull wonderful child (children?)
Dismissing that either one of you are having an affair (have you asked him?) it does strike me that this is more about re-discovering who both of you now are as 40yr olds. Can I urge time? Can you both agree to compromises over the next year or so to see if you can re-discover each other? Perhaps separate bedrooms would be a start, (my DH snores terribly and we keep different twilight hours so it works really well for us), separate bedrooms doesnt mean the death knell of a marriage. Perhaps discussing things like sex, and how for now its just not happening, BUT the thought of never having sex again for the rest of your lives is unthinkable, so a 'take a break' approach but both with the same goals n the long term?
What I think I find sad is that, you both probably want to get to the same destination. Probably still be with each other but being close, with easy conversation, having fun together as a couple again, Enjoying your DCs success. etc etc But have just found yourselves where you are and cant seem to find common ground out. You are different people to who you both were 20 years ago, but short of alcoholism, affairs, its really about discovering who you now are. Youve been a mum for so long, who are you going to be next?
Be selfish. Look outside for things that will interest you, hobbies, etc Let DC settle into Uni and adjust to not being a FT mum any more, and in the meantime, try to be as open as possible as you can with DH. Maybe even tell him that whilst you both seem terribly unhappy at the moment, youd possibly prefer perhaps to take time to re-adjust to your new ages and roles, than trash the whole marriage and possibly regret it. A great start is your acknowledgment that hes apparently as unhappy as you are. Youre clearly prepared to listen, and perhaps telling him just how unhappy and how close you are to seeking separation advice, may get him to turn his listening ears on too.
Q for both of you,if you could fix this and be happy, would you stick it out? If you both answer yes, then both of you will find no end of compromises to get through this patch (for that is what it is) and Im sure will be a far stronger and far far happier marriage coming out the other end. You must tell each other the things that really do wind you up. It can be so many silly little things, like sitting on the side of the bed to put socks on making the mattress shake and waking you up, or feeling like you have to lie in darkness because you dont want to disturb DH by putting a light on to read etc etc etc I know people talk about a date night. But Id just call it a no telly night. Tell me all the things I do that irritate, then Ill tell you yours. Its just one night only. The world isnt going to end. There is going to be sex afterwards. Im not going to ring your mother and get arsy 
The point being, its how do we be happy again, and being happy is about addressing the stuff that makes us sad, distant or annoyed.
I worked with a lady once, who was beside herself at her DC going away to Uni. Sobbed and sobbed in the office for weeks. (She was shocking 
) Anyway, come first christmas end of term, the DC returned home for 2 weeks. She went demented. "OMG! Ive loved going home to my clean kitchen, just how I left it, and now Im going home and have to start tidying up again after the little bugger..whens he going back?!!!!" Serious serious U turn.
And one other last thought. You know those old old couples who sit in the park holding hands adoring each other and inseperable in their twilight years? Theyve been through exactly what youre going through now.
Some may say, they were lucky enough to not fall out of love with each other at the same time during their marriage. Some may say they were lucky enough to not have enough money to split up during their marriage. But every one of them will have been where you are now. Sending 