You went away and left me lonely in success= Lover left him when he gained success
Understand, you may just save me from despair= Lover could save him from despair if he took him back
Take this hand, you'll see right now there's no-one there= He has no heart since his has been broken
You laugh at me as I fall= Lover has no regard for his feelings & makes fun of his sadness
Armed with love, I could save my heart, But on my own I just can't make it, I'm too weak to fight, so take it= I could save my heart from being broken but without you I to weak to make it so you won by breaking me heart.
But instead I watch you die ??= might be talking from the lovers perspective that instead of being with him he watched him die inside, or he could be talking that he love again if he could change himself but instead he watch himself die inside