Sung by Asgore, referring to Sans as the other man. She's "gone" because she locked herself in the Ruins. I found this video and loved it, so here it is.

I was fine with the men

Who would come into her life now and again

I was fine, cause I knew

That they didn't really matter until you

I was fine when you came

And we fought like it was all some silly game

Over her, who she'd choose

After all those years, I never thought I'd lose

It's over, isn't it?

Isn't it?

Isn't it over?

It's over, isn't it?

Isn't it?

Isn't it over?

You won, and she chose you

And she loved you

And she's gone

It's over, isn't it?

Why can't I move on?

War and glory, reinvention

Fusion, freedom, her attention

Out in daylight, my potential

Bold, precise, experimental

Who am I now in this world without her?

Petty and dull, with the nerve to doubt her

What does it matter? It's already done

Now I've got to be there for her son

It's over, isn't it?

Isn't it?

Isn't it over?

It's over, isn't it?

Isn't it?

Isn't it over?

You won, and she chose you

And she loved you

And she's gone

It's over, isn't it?

Why can't I move on?

It's over, isn't it?

Why can't I move on?