This seems to be discussing a loss of life.
With lines like...
"And looking at our pictures on the fireplace
Wishing it was me instead of you"
It sounds like maybe this girl was killed in a car accident or by a disease.
He's reminiscing on the good times they had together and wishing she was still with him. He eventually comes to a place of resolve realizing she's looking out for him because he's not feeling well (probably guilt and sorrow for losing her).
This seems to be discussing a loss of life. With lines like... "And looking at our pictures on the fireplace Wishing it was me instead of you" It sounds like maybe this girl was killed in a car accident or by a disease. He's reminiscing on the good times they had together and wishing she was still with him. He eventually comes to a place of resolve realizing she's looking out for him because he's not feeling well (probably guilt and sorrow for losing her).