Wow. Sounds like you've been through a lot! I know it doesn't feel like it now, but I think it is for the best that the relationship with Heather has come to an end. What you had was not a healthy relationship at all. Even if she stopped physically abusing you there was still the element of control throughout. And being completely dependent on someone else and shutting everyone else out is not good at all.

I hope the counselling helps. And with the pre degree course you should be able to find some new friends who will hopefully help to distract you a bit from her.

I know it is hard but you can get through this. If you think sending one last message to say goodbye will help bring some closure then I would go for it. I've done similar and it helps me move on.