I think a lot of the stuff said here is probably pretty accurate, but a lot isn't.
I think the song is about someone he loves but he's unsure of whether to let this person into his life or whether this person loves him back.
Evidence (How I interpreted the song):
First, the fickle thorn. I think the thorn is supposed to be a metaphor for his love. In case you don't already know, something that is fickle is something that isn't constant, instable, or changing. His love for this person fluctuates, because sometimes he isn't sure whether to put his head or heart first.
Due to past "love"s he knows that if he thinks about it too much he's eventually just going to let it slip away, hence the line "Am I ever gonna let you in before I see it coming?
Systematic in decay" where the question indicates he isn't sure whether he's gonna let the person into his life and the "Systematic in decay" indicates that the situation gets worse as time passes by, since he continues using his head more and more and thus will probably let the person slip away.
"Maybe my seed found purchase in your soil" - Perhaps his love has started developing feels for him too. Maybe his love has taken interest in him.
"I lay awake stuck by this fickle thorn
Instead of tending to it's prick I lay infected" - He simply watches as events in his love life keep unfolding and he takes no steps forward in terms of his love ("Instead of tending to it's prick [...]").
I think a lot of the stuff said here is probably pretty accurate, but a lot isn't.
I think the song is about someone he loves but he's unsure of whether to let this person into his life or whether this person loves him back.
Evidence (How I interpreted the song):
First, the fickle thorn. I think the thorn is supposed to be a metaphor for his love. In case you don't already know, something that is fickle is something that isn't constant, instable, or changing. His love for this person fluctuates, because sometimes he isn't sure whether to put his head or heart first.
Due to past "love"s he knows that if he thinks about it too much he's eventually just going to let it slip away, hence the line "Am I ever gonna let you in before I see it coming? Systematic in decay" where the question indicates he isn't sure whether he's gonna let the person into his life and the "Systematic in decay" indicates that the situation gets worse as time passes by, since he continues using his head more and more and thus will probably let the person slip away.
"Maybe my seed found purchase in your soil" - Perhaps his love has started developing feels for him too. Maybe his love has taken interest in him. "I lay awake stuck by this fickle thorn Instead of tending to it's prick I lay infected" - He simply watches as events in his love life keep unfolding and he takes no steps forward in terms of his love ("Instead of tending to it's prick [...]").