A recent presentation by leading change psychologist and usable technology evangelist B.J. Fogg claims that trying to boost our motivation to learn when we are struggling might not be efficient at all. He suggests that we should rather be smarter in using what we have, rather than trying to change it.
Fogg's starting point is that motivation is variable - at moments it is high, at moments it is low. Because of that, his recommendation is that when motivation is low, learners should engage in easier activities. An example could mechanically adding a newly encountered word to your vocabulary book, or quizzing a couple of words, just to tick of the daily exercise. When motivation is high, however, one could engage in more challenging exercises like reading news articles or books, watching movies, or doing some exercises, for example with DuoLingo. With such separation, you would still maintain your habit of learning every day, but will avoid the annoyance of forcing yourself.
Fogg has also some ideas for what you could when highly motivated, to help yourself when you're not. He summarises this in three priorities:
The first priority is structuring your future learning. To do this, whenever you have high motivation, try preparing some easy exercises that you can do whenever less motivated, be it finding sites with jokes, memes or captioned image galleries, or anything that you might find interesting in the language you're learning. This way, whenever your motivation is low, you can scroll through interesting content, encountering only limited learning challenges.
Another priority to do when motivated is to reduce barriers to future learning. You can do this by making the exercises you might have prepared easier to use, like having them available offline on your phone, or ordering them so it is easy to know where to start when you find yourself with some idle time.
Last, but not least, you can increase your learning capacity by coming up with new types of self-made exercises. Our language communities might be very useful to get ideas for that.
In general, in moments when you find learning a language difficult, try to remember that you're not alone in the struggle. There are so many others that have had the same experience. And many have managed to overcome it. Why not learn from them, with their help?