A good lens for my Nikon?

I have a Nikon D5100 and it is perfect for what i do, i really love it!
But sometimes i like to set it up on the tripod while making videos so that i can be in them (such as riding my horse) I still only have the 18-55mm lens it came with but the trouble is i can either manually focus it for videos meaning the subject can't move without me to refocus it, or auto focus which the same applies really!

Is there a lens that is able to pick up on what is moving or what i originally focused it on (like the cheaper handheld cameras seem to do) or is this just a con of this camera?
Many thanks! X

Added (1). AHh thank you! Sorted it!

I'm not too sure, have you had a good look at the manual to see if there's a video focus setting? It doesn't sound right that you would need to keep refocussing. Just looked it up and you need to be using the AF-F servo continuous auto focussing setting! Hope this works. There's a lot of info if you google 'Nikon D5100 video focussing'

Most dSLR cameras have to be manually focused in the video mode. Nikon's auto-focus does work in the video mode, but very slowly.

Here is what people add to their dSLR cameras if they want to use them as video cameras (not usually recommended - if you want to shoot video, buy a video camera)

http://mauromedia.com/cameras/dslr-shoulder-mount-mauromedia-tech/