Depression or anxiety. I am just 21!

I don't know what is it a usual mood swing or depression. I remain most of the time alone in a public gathering.i most accompany a single friend and avoid being in a group. I fear a lot to speak in public. I suspect if someone observing me constantly which makes me extremely nervous. It has been stucked in my mind that I can't write.. I have forgotten how to write.. Yes as a result of my brain functioning yes I can't write now... One day before exam I can't sleep.. When people advice to have a proper sleep of 7-8 hours in exam.. I keep struggling to sleep and ends up getting sleep of 2-3 hours.. Which awards me lesser marks.. Can anybody save my life.. Please.. Can someone advice me what should I do at this stage.. I am just 21