Motivation (and discipline) comes and goes, even for successful people that are truly passionate about the things they do. (There are studies that self-discipline is actually quite similar to a muscle that gets increasingly tired over the course of the day. Thus, it's easier to get something done in the morning than in the evening when you have probably already forced yourself to do multiple things.) That's why you need to make the things you want to do continuously – in your case: learning – a habit, i.e. a daily or weekly routine. If you make it a habit, you won't need motivation anymore and you won't even question whether you should learn right now or rather do something else. You will simply do it. If not, the habit will actually make you feel bad by depriving you from various hormones that make you feel happy. So, in this sense, a habit is quite similar to an addiction – but in a good way.
For further reading you might want to look into Charles Duhigg's "The Power of Habit". If you're a procrastinator, "The Now Habit" by Neil Fiore might also be of value to you.
Update: Several other suggestions made here (such as getting rid of the internet or using a buddy system) can be interpreted in terms of getting rid of a bad habit or as to how to implement (i.e. learn) this learning habit exactly (a buddy system is one way).