What can be done about wanting to change things in the present that you can't? I've practiced meditation, and a lot of it seems to be about not dwelling in the past, or waiting for a future which may not arrive as we expect.

But what about the present?

The deal is relatively trivial or trite; I want someone to say something: apologise. Actually, any sane person would want the same, given the (past) behaviour or this person. All the same, it seems of overwhelming importance whenever I let my thoughts wander.

And while it is pretty trivial, when my thoughts do wander in this way, I believe that everything in my life is great just as it is, except this, which is making me want to give up completely.

It seems a little wrong to say just don't think about it. Is there any other option?
There is only now, the present moment. There are thoughts about the past, fantasies about the future that are arising in the present moment.

What must be done is to become aware when such a thought do arise, and to observe that thought, objectively. Without investigation or analysis, without reacting, by simply turning, holding and sustaining your attention to what has just appeared, just at time of its duration.

You will realize that when you do this, the thought will simply disappear, it will vanish.

Ultimately it is only a thought, nothing more. The problem is that we create more out of it. The desire to control what is out there will only lead to your own misery. Let it go, give it up.

Have you ever heard about satipatthana / vipassana ?

This should help you, friend.
http://en.dhammadana.org/vipassana/what_is_it.htm