I don' t fold my thumb, but I do let it relax down into the palm when I shoot. With your arrows consistently hitting left, I' d say you' ve got one of two problems, or maybe a combination of both, in addition to what AK said. Either your arrows are overspined for your bow OR your anchor is putting the nock of the arrow to the right of your dominant eye.

The solution is to try a lighter spined arrow and/or play with your anchor. When I switched to 3-under recently I had to move my anchor forward, out of the corner of my mouth over the second bicuspid tooth to a point on my lip over the canine tooth to get the nock directly under my eye.

This is assuming that you have determined which of your eyes is the dominant eye. If you' re shooting right handed with a dominant left eye, that will also make you hit left until you learn to make the adjustment to shoot center.