I am currently experiencing this. I have managed to overcome my toughest challenges but getting overwhelmed and helpless against challenges that are comparatively much much smaller. Challenges that are more like difficult small knots, as you untie one you end up creating another. In the beginning you are not much bothered, you give it attention and try to get it all sorted out quick so you can move on with your life. But it doesn't go away completely, it just keeps evolving into something else and swallows up all your time.
You look at your life and it's all perfect except for that small knot. You wonder why it just had to be there; why it doesn't want to go away. It makes you sad and demotivated. You try to ignore it and move on with your life. And it seems to be working better than giving it attention. Now you can do something more productive and not have your life shutdown. But once in a while you still remember that it is there, and the sadness returns.
Luckily, I have some books in my library that are specifically for moments like this. Books about the struggles of the people I most admire. So yesterday, I read from two of them -- Write Good Or Die and Mark Twain: A Biographical Summary. And as expected I got a very good lifting from them, especially the Mark Twain's one.
Fill your days with steady hard work towards a mountainous goal. Progress will be slow but always remember that slow and steady still works. It still wins the race.