You are beautiful and smart and gracious and generous and your marriage was wonderful and you raised beautiful children.
Please stop second-guessing yourself.
There are things about all of us that we could improve. You have used this time for self-reflection and discovery, you have emersed yourself in wonderful hobbies and pasttimes, you have carried on with your work, you have moved and rediscovered the joys of city living, and above all you have tried to be a rock for your sons, in the face of your own pain.
There were things that we might have done differently in marriage, time off we wished we had taken instead of slogging through work, time we wish we had taken to care for ourselves, nurtured our own souls, restarted our own batteries, time off to have gone on romantic, get back in touch with each other weekends. Maybe there were bad choices made, in career, in homes, in friends, in dealing with family, here and there throughout a life and a marriage. That's true of anyone's life, there are left turns you could have taken instead of right.
Would any of that made any difference in the grand scheme of things? I think not. If we believe any that we have read and observed about midlife crisis, it is that it is a date with the individual's own destiny, and the seeds were sown long before we ever knew they existed.
So yes, use this time for you, to get back in touch with you and your God, to reaffirm for yourself that you are God's child, that He loves you and His commitment to you is unwaivering. Focus on that. Give your husband and his pain over to God, and release yourself from these feelings of doubt.
