Yep, definitely go to a 5 spot. Robin Hoods get pricey!
Like GWH said, practice out to 30 and 40 yards when you can. Your 10 and 20 yard shot will seem like a piece of cake.
One thing that may also help is shoot one arrow and make it count; just as if you were shoot at an animal. More often than not, you get only one shot, unless you're Vicki, and that first shot has got to do it's job.
Antoher thing, don't just shoot for the inside of the ring. Stick a golf T in the center of the ring and shoot for that. Concentrate on that itsy bitsy spot and not the ring in general. You'll find your groups will really start to shrink when you shoot for a specific spot, and not just a general target.