First, there's nothing wrong with snap shooting, as long as you are able to consistently come to full draw. The problem with snap shooting is that some people find it increasingly difficult to come to full draw, and they end up with target panic. If you don't have that problem, then maybe you shouldn't try to fix what ain't broken.
Second, there's nothing wrong with holding at full draw. I'm pretty sure your problem is that you are relaxing your back tension at full draw, as was mentioned. Either that or you haven't figured out how to use back tension yet, which is also common. These are the main problems with holding at full draw. There are two ways to solve this problem:
Use of the dynamic release. View Masters of the Bare Bow 3, Rod Jenkins's section, and Moebows videos on YouTube. The goal here is to gradually increase back tension at full draw until the arrow is released. There isn't much, if any, continued increased draw of the arrow at full draw, but there is a small movement of the elbow in an arc around the spine; what some people call the bottom of the "J," and is the expansion phase of the draw.
Use of the dead release. Somewhat less popular, this method is used by Rick Welch and his followers (which include me). In this method, you hold at constant back tension at full draw until the arrow is released. I have to have some gage of this, or I lose back tension. My gage is to have the string touch my eyebrow at full draw, and I try to keep the same pressure of the string on the bone under that eyebrow until the arrow is released. I also have to remember to continue to push with my bow hand.