I don’t think you are being overly sensitive. You had a tough beginning. You had to leave your parents at age 2 and then leave the relatives at age 4. As a result, it could be that you developed a distrust of attaching to anyone. On top of that, your mother has had a hard time and hasn’t always been warm or supportive. Hitting and slapping is not a way to make a child feel safe. It’s simply not okay.
It makes sense that you are cautious about getting close but it is definitely something to work on. Not everyone you befriend or love is going to leave you. In fact, acting like someone will leave might make them do just that. We do tend to get what we expect.
If you can’t let yourself develop close relationships, I do suggest that you make an appointment with a mental health counselor who can help you both understand yourself better and can guide you as you work on being more comfortable in close relationships.
I’m very glad you wrote now, when you are 14, instead of waiting. Waiting on issues like this one tends to make it more difficult to fix. Getting into some therapy now will help you have a much happier social life in school and will set you on a more positive course for relationships in your future.
I wish you well.