My case is not that different from Spiro's.
I got interested in game design as a child and spent lots of time designing tabletop games with my brother. Sometimes we would create the board and pieces with paper, other times we would use materials from existing games to create new games and other times we would play existing games with new rules, or combining two or more of them... or just writing a compilation of rules that would never be used. Really, just creating a set of rules was fun enough, I didn't need any extra motivation. Later, as a teenager, I began playing roleplaying games, and of course I created lots of additional content and house rules, and wrote a couple of brand new rule sets.
When it comes to programming, well... the case is quite similar. I just got really excited about printing stuff to the console and rendering stupid geometric shapes in the screen and I didn't need any extra motivation.
So, my point (and I think Spiro also says the same) is that you seem to be focused on the result, not the process. Looks like you want to learn game design because you want an awesome game to exist, not because you want to design games. If it's something that you actually enjoy, it should come naturally to you to do it. I bet you don't need any external motivation to play videogames, you just play them for the sake of playing them. And most of the people who is passionate about creating videogames creates them just for the sake of creating them. Thus, I think Spiro has a valid point when he says:
If you don’t answer in a way similar to these, it might be a hint that you should have a backup plan in mind.
To me, these questions you are asking are frankly bizarre. Motivation to do game design/development? Why would I not be motivated? If I needed help getting motivated, I’d pursue something else (whatever I don’t need help being motivated to do). It just seems bizarre to me that one would do anything that is not itself motivating.
That being said, also has happened to me to feel unmotivated when I pursued a goal that was way too high for my current skills, but I felt motivated again when I gave up and went for a more achievable one. I have no idea of your current skill, but seems to me that you have little knowledge about game design and programming, so perhaps you should drop Unity and Unreal Engine by now and do some beginner tutorials with an easier tool (never used any of them because I've never been interested in designing video games that much, so I can't help you very much, but I think game maker has some good tutorials for beginners and is not that hard to use). And of course, most of the experience designing tabletop games can be used directly in designing videogames, so perhaps you should start making some pen and paper stuff, or "modding" settlers of catan .
Because we're talking about game design, not game programming nor creating game art, aren't we?