It is a very common but seriously topic for all language learners.
Two years ago I started to learn English. Be honestly, I din't have a constant motivation from the beginning.
1. My original motivation is to use English in my workplace, because I word for a Germany company's branch which locates in my city. Absolutely I wanted to find some new opportunities out of my company at this time.
2. It just took me a very short term, about six months, to reach to intermediate level. It means that I could read some materials by native speakers, and I could communicate with native speakers with less misunderstands. Meanwhile, I found language skill was not the key to get a better opportunity. What I benefited from my learning process was that I could absorb many useful information by reading English materials, not just related to my work. For example, I read some books about psychology and trading. I really benefited from those books. At this stage, my motivation was that I want to make English ability as my second nature.
3. Recently I focus on vocabulary improving and pronunciation. It is possible for me to communicate more with foreigners. It seems that related opportunities are coming toward me.