Okay, so here's what I think:.
The chorus makes me think that Derek is saying that it feels like everything is getting colder, maybe he's talking about the weather, but I think he's saying colder, like darker, more depressing. And "I'll have you know I'm scared to death, that everything you said to me was just a lie until you left" probably means he's thinking that everything that, most likely Katie, said to him, such as "I love you" or "you're the only one for me", was just a lie until she left him. And he's saying that everyone's listening just to see how he sounds, and probably not even getting the meaning of the song, which probably is him saying that no one understands what he's going through at that time. And he's expressing himself through a song, and the lyrics are basically describing his breakdown.
That's what I think. This song is just *amazing* in every way, the beat, vocals, lyrics. Everything, amazing. Mayday Parade is just awesome.