Yes, you can move on – and should. This isn’t doing either one of you any good. You and your mother may not be co-dependent but it sounds like she may have a dependent personality disorder. First she was dependent on your dad. Now that he’s gone, she’s put you right into the slot in her life that your dad used to fill. She has been so indulged in this way of operating that she may be terrified of being on her own. She may be in a prolonged grief reaction, partly grieving your father and partly grieving the life she had. And she may be seriously depressed.
I don’t have enough information to make a diagnosis. But I do have enough information to see that you’ve been gradually seduced into thinking you are stuck. Your mother has you convinced she can’t live without you. The rest of the family is guilting you into staying put because they are happy to not have to deal with the situation as long as you are there.
Ask yourself realistically what the worst case scenario would be if you announced that you were going to grad school in the fall and everyone should get used to the idea. So what if your mother gets angry. No one ever died of being mad. So what if she doesn’t go shopping. At some point, she’ll get hungry. She’ll either order takeout or she’ll call on one of the other relatives.
To handle the anger: If she starts raging, you could just say something like, “I’m sorry this upsets you but your anger won’t change my mind. If you can calm down, we can talk about it but if you can’t, I’ll leave until you can have a reasonable conversation with me.” If she calms down, then have your conversation. If she doesn’t, then state calmly, and clearly, something like, “I see you’re not ready to talk to me. Let me know when you can” and go about your business. The key is not to either add your noise to hers by being angry or to respond with guilt. Stay clear and firm but kind.
If the rest of the family accuses you of abandonment, you don’t need to feel guilty. You can calmly just point out what everyone knows – that your mother is fully capable of caring for herself. Remind your brother and whoever else is offering an opinion that if they are so worried, they are welcome to take a turn at being caretakers, that you’ve done it for a very long time.
Finally: Please get yourself into some therapy. You need someone to help you deprogram yourself. As you pointed out, you’ve been part of this drama since you were 12 years old. It’s going to take more than a couple of helpful hints from me to help you break out. You need someone to encourage you and to provide you with practical advice along the way.
I wish you well.