I agree with Lsweetie, that it is a coping mechanism. When emotions are too painful this is like a reflex to seperate yourself from them and cope better. Working with your therapist on tolerating and feeling your emotions and working through them will eventually get you to a place where you won't need to dissociate. And SI does stop the dissociation and the release of the endorphins improves your mood.

I'll bet you have a load of stored emotions? Working with your therapist on unloading these will be really helpful.