Some of us simply can't straighten our arms completely. Mine isn't as bad as yours, but it's just the way that I'm built. I tried for a while to straighten it since the straight arm is a bit more repeatable, but almost injured my elbow. I even had to lay off of shooting for a week.
If you are getting consistent draw length on every shot, don't worry about it. There are some advantages to a slightly bent arm.
In fact you are probably better off focusing on absolutely consistent draw length than on worrying about your arm. However, if you really want to learn to shoot with it straighter, the blank bale is likely the place to do it. Just get up close and focus your mind completely on keeping your bow arm straight for a few thousand arrows. Then when you think you have it, put up a big target and shoot it at 5 yards. If you don't have any problems with that, step back a couple of yards. Continue that until you hit about 30 yards and then begin reducing the size of the target.
Hope this helps,
Allen