I think you need to work out why you want to leave, when you saying there's no reason. Otherwise you'll end up the same in your next relationship, or even when you're alone.
After 20 years or marriage, it's never going to be the same as it was at the beginning. And you made your marriage vows: 'for better or for worse, til death do part' or whatever it was you told each other. Not saying people shouldn't divorce or leave a broken relationship, but you did kind of promise to at least try and make it work. Not just throw in the towel when it feels a bit meh.
So at least try some marriage counselling, or individual counselling, and get to the bottom of why you feel the way you do, and see if anything can be done to fix it. Its pretty common for couples to drift apart, especially after raising children. You spend 20 years focussing entirely on the kids, working together as a team for them, that you can all too easily completely lose focus on each other. And then the DD get their independence and you're left looking across the room at each other wondering what you're doing there.
Maybe, just maybe you can get that spark back, if you both put the effort into rediscovering the relationship. Who knows, it coukd be like starting all over again together with that magical new relationship honeymoon feeling?
Or just walk away from something that's decent but no longer amazing, just because you can, without any effort. But be ready for your quality of life to plummet - with only one income rather than two combined.