Crochet is a wonderful physical manifestation of achieving calm and mindfulness because it requires focus and attention. When you are following a pattern and counting stitches, it's hard to think about anything else. That's good, that's what leads to a clearing of the head and achieving relaxation. Challenge yourself with a pattern that you think is too hard -- the extra effort and concentration put into the project will enhance your abilities to achieve focus and mindfulness. But, at the first sign of frustration, put the project down! Nothing is achieved by added stress or disappointment. Come back to it when you're ready or abandon it all together -- it is the experimentation that is valuable in this setting, not the finished product.
This video has a breathing exercise for preparing to relax and crochet as well as a discussion of the sensory nature of working with yarn.