Man, I cannot believe I forgot this one!
Many years ago, I was very unwell and had to go into hospital for major surgery and knew that I would not be able to see my DC for several weeks. The surgery was huge and I was warned that the outcome may not be good.
At the time, one of my DC who was already diagnosed with ASD, was really struggling with school, life, the whole world and seemed completely bewildered at the thought of not being able to see me for such a long time, as well as the fact that they would have to stay with their father, my ExH, for so long.
As I dropped the DC off at their father's house with their suitcases, the DC were in bits and I was trying to hold it all together.
As I drove away, I could see DC with ASD trying to break free from dad and screaming for me to 'come back mummy' and Will Young's 'Who Am I' started playing on the radio...
Who am I to tell you, that I will never let you down?
That no one else could love you, half as much as I do now
Who am I to tell you, I'll always catch you when you fall?
I had to pull over and cry huge wracking, heartbroken sobs which rendered me unable to breathe, let alone drive, for quite some time. I can clearly remember that I was thinking about what if.... the surgery doesn't go well - who would love them like I did? Who would be there to catch them as well as I could?
Obviously, the surgery went well and we all moved on, eventually, but even now, if I hear that song, I am instantly transported back to that moment, that time and even so many years later, it still makes me well up 
This is definitely one of those songs that truly makes my heart hurt