Sorry for my English, I am not a native speaker.
I am eager to see Merlin revealing his secret to Arthur, but I must recognize that keeping it secret has saved Arthur multiple times. No villain knows that Merlin is a powerful warlock. None of them knows who are they messing with and because of that they constantly fail. Well, Mordred is the exception but, as far as I remember, he is not a villain (he will be when the prophecy comes true, if it does).
Merlin could tell his secret only to Arthur, but Arthur tends to trust undependable relatives (Morgana, Agravaine, enchanted Gwen), so the secret wouldn't be safe.
However, there are few episodes left and I hope we can see how Arthur processes the information for a while and how their relationship readjusts.
I loved the ending scene Merlin strikes back!!! Very soon...