>>12559279The key to motivation isn't found, it's earned. The reason your interest wanes is most likely because you look at what you've put down and don't like it.
But why is this? The answer is simple, you don't actually know what you want to see. Art isn't just a physical thing that every artist shits out just because the can (unless they're overly dependent on muscle memory), it's heavily molded by personal mentality and knowledge of the craft. Although you may /think/ you have what you think is a great idea floating around in your head, you don't actually /know/ what it looks like, and as such your hand doesn't put down what your mind vaguely envisions.
Your head, however, has a general idea of what /isn't/ right, and when you see it heading that way you lose the drive to continue it, because you don't know where to go from there. It's this step that will change as you progress. In the very first stages you put down a line and see, well, a line; years layer you don't see a line, you see a contour. When you see a contour, you'll not only be able to tell if it's wrong, you'll know how to fix it. You'll also 'see' what you haven't yet to put down, because you've developed a sense of knowing where you're going to take your image before you even go near the next stroke.
Eventually you will come to enjoy your own art, the biggest threat to reaching this is simply giving up entirely. It might not be today, or this month, but it will come in time.
So don't give up.