I can’t really tell without talking to you but I do have a couple of guesses. Some people are born more sensitive than others. You say you’ve had ticks ever since you hit adolescence. I’m guessing that you are a sensitive person who gets anxious more easily than most. You haven’t yet learned how to manage your anxiety, so your body takes over for you.
Believe it or not, the kind of involuntary laughter you report is one of the ways some people discharge high stress. So are ticks. So are a kind of distancing moves like what you described.
Just controlling or ignoring your physical reactions isn’t going to make them go away. You need to learn more helpful tactics for managing stress. If you could do that, you would have done so already. I strongly urge you to seek out an appointment with a mental health counselor who specializes in working with teens. He or she will do an assessment to determine whether I’m right about the root cause being insufficient coping skills for managing anxiety or whether something else is going on.
Either way, some sessions with a counselor will help you understand what happens to you and will help you learn other ways to manage your sensitivity. If you are worried about your ability to tell a counselor what is going on, just take your letter and this response with you to the first appointment as a way to jumpstart your conversation.
I wish you well.