Hello Iona, actually feeling 'sad, worthless and guilty' are all symptoms of depression as is lack of motivation such as the procrastination you describe. I can't diagnose you but it isn't necessary to feel down 24/7 to be depressed. Also depression can be experienced differently by different people, some people will find they have good and bad days.

What do you feel would be helpful to you right now? Do you feel like you could go back to the GP and tell them what you wrote here? Would you consider therapy to talk over the thoughts you've been having?