Can I make it through job probation on a part-time job at a grocery store?

I have worked at Bel Air for about 5 weeks as a Courtesy Clerk, and job probation ends in about one week. They keep pulling me into the managers office and telling me what I need to improve on, and they threaten to let me go if I can't improve. I really have been trying my hardest, but my evaluation scores are not going up and they keep yelling at me. I'm really scared, and I want to keep my job a lot :( Are they just being hard on me to motivate me to do better, or am I really doing that bad? Is it hard to make it through job probation for something as simple as a grocery bagger? I'm 16 by the way.