Re: after kids grew up looks like we're heading for divorce
You ask "Is divorce then inevitable sooner or later?" No, it's not inevitable. It's perfectly normal once the nest is empty to go through a transitionary period. When you think of it, marriage through the years of togetherness, is full of transitionary phases. First, there is moving in together (whether or not it was before the marriage) and getting to know each other's habits and how each likes things done (for example, cooking, cleaning, doing laundry) and coming up with a compromise regarding how to get the daily tasks of life done. Next, for those who have children, getting used to having a new, family member who is, at first, helpless and depends on you both for everything. After that, going through life dealing with all the child developmental changes.
Then, sometimes life can throw us curves. For example, illnesses, job losses, etc. You deal with things that come along and realize that there is no perfect marriage. Marriages and emotions ebb and flow. I recall once watching Dr. Phil and he spoke of a couple he met that were married for 50 years. He asked them what their secret was. The answer was "we never fell out of love at the same time."
While raising kids, the main focus for both of you was the kids---and there is nothing wrong with that. Maybe you both rarely (if ever) had time for yourselves. Now, the kids are out of the house, you realize just how disconnected you are from each other. It doesn't have to be that way. Start by doing simple things---going out to dinner more frequently (it doesn't have to be pricey), having a weekend getaway to a nearby city or resort.
Most importantly, ask yourself---do you still love him? Do you still get along well with each other? If so, and doing what I suggested isn't helping, then consider going to marriage counseling. It doesn't always have to be a long, drawn-out process. Sometimes (depending on your problems) one only needs a session or two.
In conclusion, do all you can to put your marriage on track. I agree with the last poster---divorce isn't always the answer and from talking to divorcees, it's no walk in the park either.
Good luck and get going on changing things for the better!