I feel so left out, what do i do?

I go to college, and there's a group of 5 of us. And two of my friends are like good friends, and the other two I hang round with more, there's this one girl who's my best mate and my other friend (lia) is always sitting next to her and going with her in partners, and I'm always on my own. And when I'm with my two friend (lia) like says she wants to tell me and my friend something but then she ends up telling my mate and then I say "ohh tell me :)" then she says "oh don't worry about it" ...... Its like she don't even care about me. I'm always feeling left out, I feel useless and worthless. I'm so depressed.