"
This is not what I intended
I always swore to you I'd never fall apart
You always thought that I was stronger
I may have failed, but I have loved you from the start
He didn't intend for them to get so entangled in their arguments that they forgot how much they loved each other. He also promised to be the strong one in the relationship, to be able to take care of any problems they would face, but he suggests ("I may have failed") that their problems got the better of him, and he is deeply sorry for that. He reassures her that no matter how much he too was bogged down by problems and arguments, he never stopped loving her through the best and worst of times ("I have loved you from the start")