I'll bite! I'm happily monogamous for 21+ years, but I see one major flaw that surrounds these questions:
That is, assuming that happiness requires another person. In fact, if someone is a less than happy person, the odds are that even with a sig other they still won't be happy.
If you're still willing to bite on these questions, what do you think about the questions themselves? You've been happily monogamous for 21+ years. What did you do before meeting your spouse to prepare yourself for this relationship? What do you wish you had done? What did you do that you wish you hadn't done?
I don't what to raise the hypothetical of your being without your spouse, but can you shift to some other singleness hypothetical? If you were single and came to realize that you weren't likely to find someone, how would that change your strategies?
As an example of the second question, I'll give one of my own changes in strategy. Since realizing that I'm not likely to find someone, I've begun thinking about trying to buy land where I can retire. True bargain land isn't a bad use of money anytime, but I'd like to consider more than just price. If I still believed that I would marry someday, I'd probably hold off on these kinds of purchases. I'd rather have the money available to make this kind of big financial decision an "our" decision with my spouse instead of a "my" decision. However, if I don't believe that I'll find someone, the best path forward may be to look for the land now and buy something if I like it. If I thought that two people would live there someday, I should consider the other person. If I'll be there alone, there's no need to consider another person's desires.
Bill