Craving to chew is from lack of iron. Dramatically increase beef in your diet- every single day. The days you can't, have a Flintstone Complete vitamin. Good luck!
Malachy, chewing also focuses your brain. So if you're in a situation where you are so bored you can't stand it, and have to remain seated, you might be chewing so you can stay alert. For example, in a class that's boring to you.
Can you bring things that are okay to chew - like plastic straws or coffee stirrers?
Chewing and picking at clothes is a sign of "nerves" (as you said). The correct term is anxiety. This behaviour can be a soothing or comforting behaviour and this may be the reason you continue to do this. Have you googled phrases as "chewing and anxiety" or "how to relieve anxiety" or similar words/phrases? Often, just knowing the reason for the chewing leads to more awareness of your behaviour and when you notice yourself chewing, you are more able to stop. Hope this helps ..