For many years it has been my habit, other than at prayer times, to keep tobacco pouched in my cheek for the whole day. If I do not do this I feel that I become absent minded and inefficient at work. Is it acceptable for me to continue doing this during the month of Ramadan, if I do not swallow it?

Keeping something inside the mouth, without swallowing, is permissible and does not
invalidate the fast. However, you cannot guarantee not to swallow the tobacco juice without which you would never have the feeling of ‘being efficient and not being absent minded’.
I suggest you reduce this habit of yours to the very minimum.