Contrary to what pop culture tells the young about getting older (with all the emphasis on mid-life crises, lying about your age, worrying about appearance, etc), I find that I am much happier as I move into middle age. I am both more aware of and more accepting of who I am, both the things I like about myself and the things I don't. At this point in time, I am who I am, and that is unlikely to dramatically change. And I am OK with that.
When I was younger, I worried much more about what other people thought, about what was "acceptable" and about making mistakes. Now, I know that I will make mistakes, but that most of them will be fixable (or at least I'll be able to recover from them). I don't care so much about what I "should" be doing, and can focus more on what I want to do, even though I have many more responsibilities (both personal and professional) now than I did 15 years ago. Coming along with feeling better about myself in other areas, I find that while I still have flare-ups of Imposter Syndrome, I am more easily able to put them aside and do what I need to do with less panic.