As far as motivation goes, there's no surefire way to force the unpredictable hormone factory of the human brain to think a certain way. But if you're looking at it from a completely objective standpoint, there are a lot of ways to improve mood, and thus, productivity.
Your motivation will directly correlate with how much you enjoy doing whatever you're doing. Script writing has never been hard for me. Although gaming videos require absolutely no script, even then, dialogue will improve if you're playing something that you enjoy. That's usually why you'll see duo-gamer channels with more than one player full of so much life, V.S. somebody like Markiplier who, despite his efforts, cannot hide the fact that he is forcing himself to act a certain way.
Inventing and throwing away ideas constantly is a clear sign of depression or a fall in confidence, in my experience. In this case, the first thing you should do is to pinpoint the feelings that arise when you lose the drive to finish a script. Are you bored with the idea? Do you just not have the energy? Do you think you're not conveying your thoughts properly? Would you rather be doing something else? If you're unable to figure that out, it's likely due to depression, in which case the only solution is to improve your mood.
Do not, and this is pretty critical, do not force yourself to finish scripts unless your livelihood depends on ad revenue. Even in this case, it might be better to find another way to support yourself while you get out of the slump in question. It's like other have said already, if you don't feel inspired or motivated, your work will suffer as a result. Not only that, but you'll only worsen your mood even more.
If you're looking to improve your mood, focus on your health, or simply doing things that you actually enjoy, while not vegging out too much. Socialize more if you're the extroverted type, spend some more time alone if you're not. Exercise is a great way to generally improve your mood, as is improving your daily schedule. When you start to optimize your life, eventually your brain will crave a creative outlet again.
If you simply don't enjoy the type of subject matter you're creating now, make a switch. Start addressing things you're more interested in, or change your writing process so that it better reflects your personal preferences. Try recording some improvised videos without a scrip, even if you have to throw them away or edit them heavily. Just move towards something that you can enjoy, rather than forcing yourself to do something you don't.
