How can I make better progress on my long-term goals?

I have the curious affliction of stagnating, often for very long periods of time, on long term goals. That happens even when those goals pertain to pursuits I enjoy. This pattern has me confused and somewhat alarmed, because I know that these long term goals I have set for myself will be the most meaningful for me to accomplish. Although I see the great value in skill-building for a new career, learning to play the piano, learning a new language, and so on, I cannot seem to get myself to take the daily, repeated action required for more than a week or two. That happens, despite my applying GTD and breaking down the larger task into manageable pieces. My neophile personality simply takes interest in something else, and I miss a day (then two, then three) of taking action, preventing me from ever establishing an activity as a habit. How can I break this cycle of mediocrity, so that I can really start making progress on long term goals?