Maybe it is a beginner's thing. I, too, am more enthusiastic when I begin something than when halfway through. But, because I know myself so well, I won't let myself quit. That's where dogged determination comes in, or maybe self discipline. Just plow ahead and make yourself finish first one thing and then another. The satisfaction that comes from that will help carry you the next time.
When I first began, for instance, I loved cutting. I found great satisfaction in measuring and cutting precise pieces with a mat, ruler, and rotary cutter, which although I'd sewed for years, was new to me. Now cutting is a chore and I don't enjoy it much, because I want to get on to the next step -- piecing -- which is more fun.
It's all a process and if you approach it that way -- as a process, which has steps, from start to finish -- you may find you can complete what you start. Good luck!