It would be very rare for your daughter to have learned to be mean by 11 months. She is not doing this to hurt your feelings. She is expressing her needs in a negative way in a cycle that needs to be broken. First step is to stop thinking of her as being mean. Think of her as trying to tell you something and not knowing how.
Do you have definite boundaries for her? What do you do when she slaps or bites you? Do you calmly tell her "no biting," no slapping" and distract her by giving her something else or by dealing directly with what has caused her to hit or bite? Since slapping has become frequent, I suggest that you separate her from whatever is making her unhappy. Take her to another room, for example. Hold her hands so that she can't continue to slap you. Hold her mouth away from you. If you have to, put her down and walk away.
Temper tantrums, hitting head on floor. Go across the room and wait for her to stop. I found staying in the room, sitting on the floor near to my granddaughter helped. Once she stopped her kicking and screaming she would crawl over to me and I would hold her while she cried. If she will allow you, put a pillow under her head.
I suggest that she's aware of your guilt and uncertainty about leaving her. She's wanting more attention from you but perhaps you're coming across as too needy of her reassurance and she's overwhelmed. Perhaps you're trying to hard to make her happy. I suggest that when she rejects your attention that you just walk away. Be careful to not show your disappointment.
Because you're feeling guilty, perhaps you're hovering too closely and this is her way to get her own space. Have you had a relationship with a chld when you were a child or an adult that needed you too much? And perhaps this caused you to feel overwhelmed or crowded? I suggest this as a possibility. Getting a handle on your guilt feelings will help you parent much better.
She probably is mad at you for a variety of reasons. But trying to figure out why can be counter productive. For one thing it increases your feelings of guilt which hampers your ability to be less emotionally involved in her feelings. Her feelings are hers and yours yours. When you can deal with your feelings her feeling will seem less personal and you can be more active as compared to being reactive. I'm not saying this clearly, I know. I can't think of the right word to describe this look at boundaries.
Try coming home and not showing how excited you are to see her. Let her come to you. You cannot directly change her behavior but her behavior will change when you change yours.