How do I accept never seeing my life's work finished?

You've built things that fly, and the pace and secrecy of the work mean you may never see your life's work fully realized or even acknowledged. That's a particular kind of ache, to pour yourself into something and not get to stand back and see the whole of it. Sitting with that is hard, and it's fair to name it. But finished and seen are not the only measures of a life's work. What you did is real whether or not you ever watch it completed. The knowledge you added, the problems you solved, the younger engineers you steadied, all of that is woven into things that will outlast your view of them. Much of the most important work in the world is done by people who never see the far end of it. Find your meaning in the craft itself and the people you've shaped, not only in some final unveiling that may never come. Look at the specific problems you cracked, the moments you knew you did it right. Let those be enough, because they are the true substance. The cathedral gets finished by hands that never see the spire. Yours are among them, and that is not a small thing.