I'd often feel something of a failure for not having a career, but gradually accepted that my role as a mother was also important, so I guess this is how I came to define my worth. This is probably also why seeing my motherhood role coming to a close has left me feeling like my usefulness is coming to an end.

No dear, you still have a lot to offer. This is actually more common than you realize. You will definitely find the next phase you're going and fulfillment on the way.