建议：1. 参加学校活动和体育锻炼；2. 多和朋友、老师或父母交流沟通；3. 多对周围的人微笑。
(1) 开篇先用I’m sorry … 引出写此邮件的原因。
(2) 在向对方提建议时，可以用First, … Second, … At last, … 来梳理全文，从而使文章结构完整、条理清晰。
Dear Wang Fang,
I'm sorry to hear that you are in a bad mood these days. Here are my suggestions to deal with your problems.
First, you can play an active part in group activities and do sports with your friends in your spare time. You will have a great time together. And you can build your body up. Second, you should talk about your problems with your parents or teachers. Maybe they can give you some good advice and help you get out of the trouble. At last, I think you can smile more at others. Then people may think you’re friendly enough to get along with. In this way, they will make friends with you happily.
Anyway, I hope you can feel better soon.