I have the same problem. I have realized that I overthink things. I make small things seem like big obstacles. Like making a quick phone call. Yes, even that is something I put off. I have decided to lower my expectations to myself. If I manage to do just ONE of the things that I have been putting off, that day, then I am happy with myself. Dont ovethink. Remember that once you get started, you are on your way. Maybe it only takes 30 minutes to do one task. And you spend more energy agonizing over what you dont get done, than the energy it takes to actuelly do it!
But keep in mind that if you are coping with a mental illness, you just dont have the energylevel to get a lot of things done. And one might have a hard time with concentration and focus. So dont be too hard on yourself.