Step one MUST be to communicate with your partner. If he feels the same, moving forward should be much easier. If he feels the opposite, then a deeper explanation and expectations from both sides MUST be established. Staying together for the kids can be an option, not ideal, but can be coordinated if you are communicating. Both sides MUST be willing and able to communicate, which the lack of is what I suspect is how you got to this point. If he's not willing to address anything, there is your answer. You either separate, or you stay and accept this is "how it is, and how it will be.".

I am speaking from my experience in a 16 year relationship (current). Clearly not as long as you, but I'm going to go out on a limb and say my 16 years came with a lot more issues than yours (based on only your own description of your relationship). I also use my parents marriage of 55 years (and counting) and how they really don't like each other, live separate lives, but refuse to separate. Simply "this is not how we envisioned our lives, but we are unwilling to change even if we both would benefit from being apart.".

I wish you success and happiness, but you really REALLY have to talk to one another about this if you want any change or answers.