How can I stop ruminating?

Hello folks!

I'm 17 (female) and highly sensitive. Do you know that feeling when you always seem totally happy and positive on the outside, but on the inside you're constantly thinking about something? I'm always told that I'm always so friendly, positive, and in a good mood, but at home my thoughts completely overwhelm me. They're completely pointless thoughts, but they're there and won't go away. Well, at school and around people I feel really good and all, but when I'm alone I sometimes feel like I'm going crazy. Do you have any tips on what I could do about this constant thinking? I have the feeling that my brain always comes up with something new as soon as I've finished with a chain of thoughts. My problem is that my friends don't understand me. They don't know what it feels like. My family even less. They always talk to me and try to understand me, and I can talk to them about anything, but they don't understand how I feel. The slightest violence on TV makes me go crazy. Another room because I just can't stand it. Whenever I hear a touching story or see it on TV, I always get goosebumps. What can I do about this constant brooding? It's eating me up!