Hello I dont understand I bought for 4000 Eur one hour ago of Ethereum when it was going up, it stayed around the 10-20%, it started to decrease 5 minutes ago so I sold it and now cex.io just gave me 3500eur for it... wtf...

The %came from me watching livecoinwatch.. Cex.io doesnt have the accurate price ??
You get panic. Trading is not easy, actually as a new trader, we can experience those things and we cannot handle our emotions during our first months or weeks in trading but a couple of months you will find yourself and decision making finally improving. You can handle those problems and know how to hold your investment.