I think one of the biggest psychological obstacles you can have to overcome is dealing with the expectations of who you thought you were going to be. Mostly this is something you have to deal with when you're older - I don't think most 21 yr olds are given to that sort of introspection anyway, though you don't have to be a great deal older. I'm sure some of the impetus behind the mid-life crisis comes from this, but you don't have to be midlife for it to happen.
Nor is it necessarily a one-time only thing. You can have a plan for yourself and a path you want to follow, only to have it taken away from you. It can be something you worked hard for over the years, never dreaming it wouldn't happen (and consequently not having a back-up plan) and then, suddenly, you have to deal with the fact that it isn't going to happen. Or maybe at some point later you take a look around at your life and realize that, while there are many good things, you aren't where you expected to be, not because you couldn't get there but just because you haven't yet. These are the times when you may have to redefine what you want - which in some ways defines who we are, after all.
I didn't call this "reinventing" because that's somewhat a more drastic approach - and ultimately I think those attempts fail. We have certain things at our core, and those don't change. There are only so many directions you can go along the paths you have available. I could try and reinvent myself as a nuclear physicist for example, but given my limited mathematical skills it would be doomed to failure. But there are certain paths I can take, certain options open, and at various times I've made decisions about which to choose.
There's a metaphor here for writing too, of course, because often times we start out with a vision for where a story is going. Then we go along and suddenly that vision isn't working. Which is when you have a choice: you can sit there and lament what might have been and how it didn't work out, or you can go with it and see where it takes you. Which sometimes works out much better than your original might have ever possibly have.