I've been playing guitar every since I was 9 yrs old and now I'm 14 and have found that the way i hold the guitar is giving me Tennis Elbow. What is the best way to get back to playing the guitar properly
Well since there is no "trick" to correcting wrong technique from one day to another (especially if you have been doing it for 5 years) I'd say this is going to be a matter of a lot of practice and concentration.
Every time you pick up the guitar and play you should actively focus on how you are holding it and only play with the correct technique from now on. This also applies to when you are in the middle of playing: If you catch yourself falling back to your old habit, immediately stop, hold the guitar correctly and then continue playing.
If you do this properly you will eventually get used to holding the guitar correctly and you won't have to concentrate on it as much anymore.
Note: As I said this eventually does not mean in a couple of days. The amount of time this takes entirely depends on your dedication and how much you practise your guitar. But even if it takes longer than expected you should never give up, since learning the correct technique does not only affect your playing but also your health - as you have already figured out yourself.