You are only allowed to have 5 points of concentration active at one time. In order to cast your new one, you will have to cancel one of the others.
Now, take a look up on the left right under your name and you will see five little light blue boxes. Each of these represents one of your active spells that require concentration.
You can also see them on the left hand side of the screen in the "active" spell tray. And finally, you can see which of your spells have conc. by examining them from your knowledge book or from your hot buttons.
Sometimes in life, you just have to make choices...