I'm going to assume you feel that way both physically and emotionally!
Did the pdr offer you the hope that she can work with you to get things more stable so you are able to start the other thing (it was a bunch of letters in your other post) when you are more ready?
I'm sorry you feel emotionally drained, it perhaps isn't a surprise since this seems to be the first day when you are well enough to realise the impact of what happened and all that means. Self harm is a temporary fix, so it is understandable you still feel upset emotionally or realise how upset you've been now that the shock and action of getting treatment has worn off a little.
Be kind to yourself, if you want to lie around and take some time to whallow in it, I think that's okay, you haven't allowed yourself to do that - if anything you've pushed impulsively and reactively these feelings away by OD'ing.
I'm by no means saying its fun or easy or will make you feel better to whallow. Just that sometimes it is okay to allow yourself to feel without having to change it. But I get how much we want the pain to be relieved for a second, that's why we self harm sometimes, it makes sense because the pain is awful. My head changes when I'm too overwhelmed, I think it might be about finding a balance between accepting we have an illness so are naturally going to have periods of deep emotional pain, and having hope and techniques to get through those times more successfully....I'm slowly coming to the conclusion that, for me, my bad times can't be fixed but I need a lot more hope they matter, and it isn't purely survival....sorry talking too much about me...no one wants to hurt themselves, I wonder if you know why you do and if there are any longer term support. I guess your therapist will be helping you with that...
Take care of yourself.