If you want to hold at full draw, the first thing to realize is that it's a skill that has to be learned, not something that is just going to happen without work on your part. If you are used to realeasing as soon as you touch anchor, then you are going to have to overcome the urge to do that, by forcing yourself to hold. At first, you may actually shoot worse, since you are focusing so much of your attention on holding. Later, after you are used to holding, you may shoot better. Or, you may not, and instead discover that you are one of the many people who are better off snap shooting. But you'll never know until you put the effort in to make the change.
If you try really hard to hold, and still can't do it, then you may have target panic, as Jim suggests.