I think I am severely depressed and I do not know what to do?

To be more specific I am afraid I am suffering from cyclothymia (a milder version of bipolar disorder)

I feel horrible right now. Just completely lost and hopeless and I just want to scream and cry and I do not know specifically why.
But some days I am full of confidence and determination and energy like I will get X, Y and Z done by the end of today.
And I never do. But it keeps happening, this cycle of confidence and then sadness.
I do not know who to tell.
I hate telling my friends, because I feel guilty for burdening them all with it, I just want to be happy with them so I fake it and slap a big smile on my face in the hopes that will work and I wont drive them away.

My parents could not care less.
My teachers are already concerned but I do not want to come across as an attention seeker or something equally stupid.
I just want, help. I guess. But I don’t know who from. The one person I felt I could go to is leaving soon, I’ll never see them again not going into details and I want to talk to them, but I don’t want to make our last days together full of my problems.
I don’t know. I simply do not know what to do. I’m frightened, really truly frightened. I’m scared what might happen.