What does "be the one" mean?

There is not much context given about this dialogue so my answer is necessarily a bit vague.

What is meant is something like:

I've been in this situation before ... over and over again, and each time the same question. Is this the best to be hoped for? Will the the situation this time be situation that I desire.

With more context, I could give a more specific answer. For example, if she is talking to herself about a potential mate, it would mean something like

I've had these feelings before ... over and over again, and each time the same question. Is my love strong enough? Will the man this time be the right one?

If she is talking about buying a new kitten, then the meaning will change. Every word in this dialogue is vague: "here," "this," "it," and "this one." People do talk like this, but they do so in a context where these vague words clearly refer to something more or less concrete.