The best thing I can suggest is to start with that 5 minutes. Commit yourself to that. Once that 5 minutes is up get up - get a drink, do something else briefly as a 'reward' for completing the 5 minutes work and come back. It can be frustrating but in time that 5 minutes will get easier. Then you can look at increasing the time slowly.

It definitely isn't an easy fix but it really can help.