Unfortunately, I struggle with motivation as well due to extreme depression. Best advice I can give is to try your best, and don't beat yourself up if you can't meet your goals, especially if you manage to make any progress at all. Using writing as an example, writing a full chapter is great. Writing half a chapter is pretty good. Writing 200 words is significantly more than zero, and if you keep doing it, you will end up with a chapter, so it is good too. And keep in mind that doing your best can mean different things on different days. You might be able to manage more in one day than in the entire week before, but the important thing is doing your best for a given day. And as others have said, habit is a big part of it. Don't be like me and keep avoiding things just because you can't do them completely, instead form a habit of trying to get at least a little done every day.