First of all, I’d like to applaud your determination to get back into work. Unfortunately, depression is something that some people still misunderstand whereas it is a condition that needs to be managed, just like any physical condition.
First of all, have you checked out online resources such as Mind? They are very focused on employment issues for people who have a mental health condition and might be able to help you. Their website is very clear and organised in subject areas and you might want to e-mail them or phone them to receive detailed advice that suits your circumstances.
If there is no pattern to your difficult episodes, it may be that working from home might be the best option for you. Have you ever thought of undertaking graphic design work at home by working on a freelance basis? There are a number of freelance websites where small businesses post up assignments and freelancers reply with a proposal. Also, there are websites such as Remote Employment where you might find teleworking jobs. Finding an option like this may take away the worry of being able to go to an office on any particular day.