I don’t have a solution. However, here’s suggestions.
Try to evaluate her actions and words as a reflection as to her own insecurities or immaturity. Try to assess if there is any thing you can say to her to get her to respect you the way you wish. This would likely involve setting clear personal boundaries. If not, then don’t be her friend, and spend time with people who you have better relationships with. You can also try to make new friends by joining activities you have genuine interest in. I’m guessing if you find people who share your values or interests, you’ll be happier.
Also, if you have some friends you like better, ask them directly if they view you as childish. Ask them to be specific and honest. There might be something easy you can do differently. Or they might not view you as childish at all.
The mechanism you are describing about fat cells refilling vs making new cells, does not make sense. They have done studies, and the number of fat cells remain constant during adulthood, unless you undergo liposuction or other procedures. I think what you are describing is a set point weight, where your body tries (via hormones I think) to maintain its preferred weight. So its more a metabolism effect rather than physical space issue about storing fat.