Firstly, the doctor's sick note will originally have stated how long they expect you to be unable to work, so you need to go back for a review of the situation - and if things are no better, there appears to be no reason why the doctor's note shouldn't be extended for (say) another month. The doctor cannot sign you off as Fit to Return to Work until they believe you are fit, mentally or physically. However, your employer may feel that you need to see their own Occupational Health doctor, to back up or otherwise what your own GP says - but this will depend entirely on your employer's Sickness policy, usually this happens after a couple of months, not immediately. Your GP is the only person who can determine whether or not you are in a condition to resume your existing work. Go and talk to your GP and explain that you are still feeling severe stress, and ask if the sickness absence can be extended.