Am i that bad at minecraft?

I don't get it, I dueled about 20 times at GammelnHD I lost all 20 and most of the others still had 20 hearts I don't understand it 8 clicks is not enough or do the opponents just make 30 clicks?

You probably click too fast, you have to wait a certain time between hits otherwise there's less damage.

You get better with practice. Maybe take a look at a few PvP Let's Plays, there you could learn something.

There's no cooldown on the GommeHD.net server.