I'm afraid that if I tell others what's wrong with me mentally, i'll be even more excluded. What can I do?

What's wrong? I don't know what you consider "wrong" with you mentally, but you can choose who, how, and when to tell, if at all. Feelings of being excluded may especially accompany certain illnesses such as depression, because one's perceptions are shifted towards a more negative self-view. It's important not to exclude yourself from compassionate others who may understand and care about you, but choose wisely.
Choose carefully who. You tell about your mental health issues. Even those you trust most may or may not be able to handle it well. Unfortunately one can't always determine who can and can't deal well with it. Sounds like you are already feeling on the outside. Maybe you need to work on finding support groups designed to give you the kind of help and acceptance you should have. You also may want to talk to a counselor.