Write it as a letter starting with Dear and so on (this is what I did last year and I became a semi-finalist). I even noticed while looking back at my old letter that I had a few grammar mistakes because I didn't type it in Word first. Opps. I talked very natural in my letter because it is a letter! So you don't have to stress over it very much just talk about yourself like how you would to someone in person.
And therapy. I don't think that will affect you. They might ask why but everyone needs someone to talk to. It's pretty common especially if you're a teen, lots of hormones and confusing emotions you want help on. Hope this helped some. Good luck! ^^