Everything you are feeling about your mom is a super normal reaction to the situation you are both in. It probably takes a little bit of courage to go through the door and try once again to make her life a little more comfortable and pleasant. When you walk out that door you're probably sad, tired, lonely and feeling abused. At least that's how I felt dealing with a similar situation.
Helping someone you loved out of this world is one of the finest things you can do. It's humbling, serving someone who isn't and is the person you loved. There are so many tough days and contradictory emotions at play and in the end you swallow them and come back the next day.
Good luck to you and your mom.