LOS ANGELES — In a highly-anticipated testimony Thursday, the plaintiff in a landmark social media addiction trial said using apps like Instagram and YouTube when she was a child fueled her depression and anxiety, leading her to withdraw from her family. Now 20 years old, the plaintiff — identified in court documents by her initials, K.G.M. — detailed how her almost nonstop use of social media “really affected my self-worth.” “They made me give up a lot of hobbies and old interests, and they prevented me from making friends ... [and] caused me to compare myself to other people,” she told jurors in Los Angeles County Superior Court. “I just felt like I wanted to be on it all the time,” she said. “If I wasn’t on it, I was going to miss out on something.” K.G.M.’s trial is the first in a consolidated group of cases brought against Instagram, YouTube, TikTok and Snap by more than 1,600 plaintiffs, including over 350 families and over 250 school districts.…