Not to knock you, but this sentence is where I think the issue is - at some point motivation drops off and you need what I like to label as "discipline" to progress further.
The best piece of advice I heard was from someone getting awarded their 5th degree black belt in Aikido, something that takes decades to earn [I'm paraphrasing here]: Training is like a marriage. It is great in the beginning because you are learning all these new things and improving, like the honeymoon phase, everything seems to be great. But over time, that feeling will decrease a bit. Sometimes you wonder if its worth it at all. But, like a marriage, training takes effort and even years down the road you still have to work on it. Not just maintaining it, but actually working on it.
I also like a book by George Leonard called "Mastery"  that I think I've linked on HN before. In the book, he's taking the position of the instructor to explain types of "student behaviors" he's seen over the years - Dabblers, Obsessives, and Hackers (people who just "show up"). As learners, people will behave like one of these types on their way to becoming masters in an domain. Each student type stressed the same topic - dealing with when improvement "gets hard" and what steps can be done to help curb some of those issues.