Can someone regain a Beginners Mind when it has gone?

No. You'll never get it back. You're not a beginner anymore. Your definitions are what prevent you- the fact that you experience a "beginners mind" as something of its own means you hold yourself to those definitions. The mind should be in a constant state of what you might term "beginners mind" simply from knowing itself- they go hand in hand. Not just observing oneself, but the unity of knowing oneself. So I say; find yourself and you will have what you want. That's where I want to stop, because I don't think you'll really benefit from an explanation, but this is stack exchange so it's for everybody and I'll give some explanation.

What you are calling a beginners mind is a concept that you have extrapolated from something somebody once said. It was said to call attention to the particular ways of thinking by relating them to an experience we can relate to. When we are new to something, we are somewhat in awe of it because we cannot sometimes even fathom its circumference. To us, it is still infinite, in a sense. The expert, however, is well acquainted with something; to him, its circumference is well known. No mystery left there. But there is ALWAYS something more and you do yourself a disservice to ever think that you've seen it all. But really, these two are merely the two sides of a coin that you should just throw away. Hope this helps.

A few years ago, I started having an experience like "I'm finally learning the things I knew when I was 17." (I'm 55 now; no implication that it takes decades, lol.)

It's natural to have beginner's mind when everything is a literally new experience, and you're "learning". Now, beginner's mind is still natural (see soulsings' answer), but a "learner" part of you will be participating in that, in new way after new way. That may seem like you're "learning more" (see qweilun's answer), or it may not require learning at all.

For those of us who came to Buddhist thinking thru New Age thinking, this is one of the places in New Age thinking, where it's easy to get stuck: thinking that it will always be spiritual "evolution", self "development" (whatever the self is ;) and "learning principles"... all of which has value, but if we "identify" with this "forever learner", it automates itself, and can be experienced as unstoppable, even as a burden...
Whereas another thing I've noticed recently: that my most touching "lessons" are not "learning"... less and less "new information"... more and more "old information freshly present."

Somewhat aside, but that you might enjoy considering: Gangaji, (a Zen Buddhist who became a student of Papaji), sometimes says things like this, that have a beginner's mind flavor, for me: "What is at peace, what is already free, what is endless in its realization of itself, is here now, and is who you are."
In that context, there is the endless, which will never provide a "place to land", and the already here, which does not require a "place to land."

In the Mahasi tradition we don't have the idea of beginner's mind, just a continuous refinement of the process. Many times throughout my practice, I've learned that some phenomena or thought process was just that—where previously I mistook it for a deeper reality. And I continue to experience these refinements.

But to attempt to answer your question, try not to hang on to the desire for Beginner's Mind. It will come back on its own. In addition, I would say be gentle to your mind and body. Those particular helped me the most when I was struggling in my practice. Maybe it will work the same way for you, in that being gentle with yourself may allow the Beginner's Mind to return when you least expect it.