It sounds as though you perhaps have 'Social Anxiety' (about which, there's plenty of information online). Not that I'm wishing to force labels onto you, without knowing much about your situation, but the dizziness and feelings of sickness around crowds/groups of people hint at existence of the condition; even if these are not regularly experienced.
I know what you mean about unwelcomed meetings with people from the past, which may seem all-too-frequent. You're only young, so don't worry; you have plenty of time to meet new people, to make fresh starts and to have fun while still being young. Times change; if things seem bad, they won't necessarily always be.
Oh, and if there's any place to get things off your chest, this is probably it, so don't worry on that score. All the best.