I have no clue if the songs I'm suggesting below will really fit what you're asking for, but I'm giving it a go:
Situation a:
It’s a farewell situation.
Her lover has to leave home (and her) because he has to go away to be a soldier/hero or something similar
Now she sings about his leaving and their everlasting love.
She expects him never to return.
The song could also be a duet where the male lover answers that he has to go (and fight/be brave/be a hero) even if he doesn’t want to leave her.
It is also possible that she sings it, when he’s already gone.
Makes me think of the Dixie Chicks' Travelin' Soldier: this is about a young soldier going to Vietnam; right before he leaves, he meets a young waitress in a cafe, asks her if he can send her letters, then at the end of the song the little waitress learns that he died in Vietnam:
"I cried
Never gonna hold the hand of another guy
Too young for him, they told her
Waiting for the love of a traveling soldier"
I also thought of Chris De Burgh's Borderline:
"And it's breaking my heart
I know what I must do
I hear my country calling
But I wanna be with you
I'm taking my side
One of us will lose
Don't let go, I wanna know
That you will wait for me until the day
There's no borderline"
Situation b:
The female figure got a message, that her lover is presumably dead.
Now she generally sings about their love, compares it to heaven, describes his kisses and his beauty. In the end is a feeling of despair because he’s gone (died probably) and will never return.
At first I thought of My Immortal, by Evanescence... but then a kangaroo eating an apple would make me think of that song (we're talking of my favourite song ever ever ever here)
"I'm so tired of being here
Suppressed by all my childish fears
And if you have to leave
I wish that you would just leave
Your presence still lingers here
And it won't leave me alone
These wounds won't seem to heal
This pain is just too real
There's just too much that time cannot erase
When you cried I'd wipe away all of your tears
When you'd scream I'd fight away all of your fears
And I held your hand through all of these years
But you still have
All of me"
Then I went blank and had no other thought because I went back to listening to My Immortal.
No clue if that helps at all!
Kaethel