I believe this is referring to what led to Marcus's breakup with Laura Marling, and then what happened after that (his reunion with his now-wife, Carey Mulligan). "Fingers tap into what you were one, worried that I blew my only chance" refers to when they were pen pals, as kids. I can imagine him coming back from tour finding the letters he got from her and going through them, and thinking "I thought I lost my chance with you after I lost contact."
"A brush with the devil can clear your mind" means that it took doing something foolish to see that he shouldn't have been with her (Laura) in the first place. "I lost my head, but I found the one that I love under this sun" means "I messed up, but something good came out of it (I found the one that I was meant to be with the whole time).
I'm going to take a whack at this.
I believe this is referring to what led to Marcus's breakup with Laura Marling, and then what happened after that (his reunion with his now-wife, Carey Mulligan). "Fingers tap into what you were one, worried that I blew my only chance" refers to when they were pen pals, as kids. I can imagine him coming back from tour finding the letters he got from her and going through them, and thinking "I thought I lost my chance with you after I lost contact."
"A brush with the devil can clear your mind" means that it took doing something foolish to see that he shouldn't have been with her (Laura) in the first place. "I lost my head, but I found the one that I love under this sun" means "I messed up, but something good came out of it (I found the one that I was meant to be with the whole time).