Just trying to improve my English :)

Hi Guys! Today I just found myself thinking about why it's so difficult to improve my speaking... and I really don't know the answer! I mean I can write ( not very well, but I think that you can understand me) read, I'm going well about listening, but speaking is my big problem now! Almost always I know what I have to say, but when I go to say something the words just go away... I've been learning by myself, and some times I don't practice here because I have no time but now I'm back :) I'm counting on your help!