In the olden days we had sick notes in the UK. This meant if you did not feel well enough to work you saw a doctor and if he agreed you were sick you got a note and stayed off work until you were better or died.
Now we have a fit note. If you do not feel well enough to work you see a doctor and if agrees you are sick but not yet dead you get a note saying you can go back to work until you are better or die. Workplaces have to allow shorter hours and retrain people in different roles.
It really doesn't work for mental health problems as the stress of relearning a whole new job and remembering it all is greater than the original job. So this guinea pig has found anyway. And if you didn't want to leave the house in the first place , leaving it for 4 hours is the same in and out process as for 8 hours.
It is all very new so if anyone has any good ideas or links to other guinea pigs nearer the cliff than me I would like to hear their ideas.
I understood that it wasn't the going out that you didn't like but the arriving at your workplace, so maybe shorter hours with a different task and less responsibility could be a way out of your current unsatisfactory situation. If the new job is just as bad as the old one, at least you have the opportunity to do it badly. 
Perhaps our new government will make changes. Not so much out of kindness as efficiency. This 'get people to work no matter what' seems to be more of an obsession than a carefully thought out policy.
While it's obviously a good thing for most people to be in jobs providing goods and services simply getting someone into an office/factory and crossng them off the unemployed list doesn't of itself help the economy.
Aside from that slim hope I can only suggest being firmer about what you are currently fit enough to do.
I really don't want to go out at all but it is made worse by getting where I am going and it being horrid there. It takes so much of my energy just to exist and I am old and tired and the universe is run by idiots etc.
It does help the economy because instead of people claiming full wages on sick pay paid for by employers and then going on sickness benefits paid for the government, they have to live on part time wages.
Whilst researching this I found this link which does not help with the dilemma but I thought it was amusing.
http://thesatirestall.blogspot.com/2010/04/are-you-too-sick-for-fit-note.html