Complex sentence: After arriving, she was signed with the Prestige Agency and a became a model who appeared on the covers of "Seventeen" and "Brides", but by age 21, she was battling an addiction to drugs and landed in rehab.
Split into simpler sentences: After arriving, she was signed with the Prestige Agency and a became a successful model. During her modeling stint, she appeared on the covers of "Seventeen" and "Brides", but by age 21, she was battling an addiction to drugs and landed in rehab.