1. As people have said, do not bottle up your feelings
2. Being sensitive is not a bad thing, especially as you're a girl
3. You will begin to understand people more as you gain more "worldly experience". This does not mean that you will understand things by becoming older -- it means you will understand people, what they do and what's going on inside by experiencing the world. This will also help you put things into perspective (note: this doesn't mean you'll become thick skinned or insensitive, it just means that you'll have a much bigger framework/concept of life into which you place occurrences, meaning that small things have less prominence, in turn meaning that they will bother you less).