I have h.Pylori, I am 20 years old and have been treated for it. Does the treatment lessen the chance of stomach cancer? I am terrified...

Yes. In fact, the risk of it giving you stomach cancer is quite low, and drops to very very low if the bug is gone. In areas where historically most folks have had helicobacter, the rate of stomach cancer is still not high especially among the young.
H pylori. Treatment learns chance of ulcer disease stomach cancer is not associated with h. Pylori. Some can be associated with lymphoma of stomach but that is rare and treatment of h. Pylori prevents it.