I do this without realising too, it is a hard habit ro break. It defo increases my anxiety. In fact, I'm sat at the dentist having just seen the hygienist and waiting to go in and have an old filling replaced, and I'm doing it now. That is even after 2.5mg of diazepem! Wish I had advice that work, but other than to say try and catch yourself doing it and perhaps learn some yoga breathing techniques (in through nose, out through mouth and out breath longer than in breath) I don't know what else to suggest as I've been doing it for about 15 years now!!