You should ask her to set specific goals for you to meet, so you can "earn" more hours. Tell her "too slow" isn't very helpful, and you'd like to do a better job, and were not aware of any problem before this, and you want to get better.

It cannot hurt, and will show you want to improve, and that may help. In the meantime, you can look around for another job in case she doesn't follow through with you, and you need more hours.

But remember, it is in her and the company's interest to keep an already-trained employee, rather than hire and train someone new ... so she should want to work with you.