Stevie. You are going through a very difficult time. You have done some things that you regret and that you felt you were driven to doing by a force that was stronger than your healthy self. You are right. You were so consumed by the persuasions of depression that they manifested in colours and actions that you wouldn't, didn't, and don't approve of. This is incredibly difficult to come to terms with for you and for your girlfriend.
Can you say how you felt when you physically handled her? This was very wrong, as you know. What emotions were going on? There can't be excuses for physical overpowering mate, but, there can be explanations. The only thing we can ask for is that some one hears an explanation and considers it kindly and with an open mind.
It may be, that your action was SO utterly unlike you that your girlfriend simply could not believe her eyes and had a very frightened reaction through sheer unexpectedness. We can see how she might. I suspect, from your posts, that you also were surprised with yourself and don't actually blame her for her reaction? you are as appalled by yourself as she is in you?
If in your heart you are benevolant and gentle then you must first forgive yourself and come to terms with your actions. Consider talking about this here because that is a very difficult thing to do - you are your own worst enemy and disciplinarian. You must ask your girlfriend to consider who she knows you to be and ask for her forgiveness. She might be able to forgive you but she may not be able to come back, but hopefully it will give you some foundation to build a better understanding of yourself, come to terms with what has happened and tie up these loose ends and move on.
You made a mistake. It had consequences. But you can find a way to forgive yourself through understanding and find a way to move forward.
I feel for you mate. Keep holding on, we're here for you.
Lol