No matter what the overall meaning behind this song is, in my opinion "I won't ever be your cornerstone" means that he won't support someone else's beliefs anymore. He wants to stand on his own morals and values. I think the burning theme ties in here because he wants to burn down the figurative foundation of all the things he has built up until this point to disconnect himself from the pain and hopefully start over again in a more pure form.
No matter what the overall meaning behind this song is, in my opinion "I won't ever be your cornerstone" means that he won't support someone else's beliefs anymore. He wants to stand on his own morals and values. I think the burning theme ties in here because he wants to burn down the figurative foundation of all the things he has built up until this point to disconnect himself from the pain and hopefully start over again in a more pure form.