Don't hold your hands near your stomach, they should be balanced somewhere over the horses withers. Just think, where they are now, if your horse threw up his head and tried to take of, you couldn't pull the reins, because you would have nowhere to go :)
My old instructer used to say their should be a line from ear to shoulder to hip to heel - the should all be on the same vertical line. You could probably drop your stirrups a hole which might make it a little easier. He used to say imagine there was string down there or something, I don't know, its been a long time.
Also, with your reins, it might be a little strange getting used to riding with a contact if that's not what your used to. In fact, you may find the reins slipping, so reins with little notchy things might help. That way you can decide of your notch, and if you slip back a little, you know you have and correct it :)
Sit back more, but if you try and sort of get a line going like little picture in Vicizmax's post then that should sort itself out.