i haven't read all of the responses, so again, sorry if i repeat anything already said!

self injury, like any real addiction or coping mechanism, can be reused once "opened." maybe on new years when you first injured again, you realized that hey, this works! i feel better! maybe this is a good coping skill.

obviously there was a failure in healthy communication within yourself, but maybe you subconsciously enjoyed it and put it in your "tool box" to use later.

again, these are just my personal theories based on my own personal experiences. i hope you are feeling even better since yesterday!