Ich brauche dich, ich vermisse dich, ich könnte dir erzählen, dass ich dich liebe, aber es ist nutzlos, da du ein anderes Mädchen liebst! Alles was ich habe, ist Schmerz!
All that I have, is pain.
?
Ache is the same as pain, isn't it. Although you'd rather speak of a heart ache instead of pain...
Then why change the text?
Although "ache" and "pain" are synonyms they are usually used in different ways.
"ache" is generally seen in compound nouns like "headache" "toothache" etc, or as a verb in sentences like: "my back aches" or "I ache all over"
For this line I think the correct structure is the inverted "Pain is all I have" (without "that"" , but if you insist in keeping the word order it should be: "All I have is (the) pain"
It is not I who insist. It is in the text.
'Pain is all I have' would be in German: 'Schmerzen is alles daß ich habe.'
It has to do with a extra stress the word gets when you change the sequel.