Hi, Ashley.
Welcome.
I hear so many emotions in your post.
Guilt, anger, resentment, low self-esteem, wanting to do the right thing but not sure what that is.
Is there someone who can help you sort some of these things out?
Sometimes our parents are just not that good for us.
Sometimes they are perfect examples of how NOT to be.
Please take care of yourself. Stress doesn't help anything.
Your are being a reasonable grown up when you offer to take mom shopping when you have the time to do so.
She is not being reasonable when she calls you crying and enraged.
Sounds like some mental disconformities there?
I have heard it said around here that boundaries are not for the other person.
They are for the person setting them.
Decide your boundaries. Not today, not tomorrow, but think hard and decide what you will take and what you won't from your mother.
Then turn your phone off.
Peace. Keep coming back.