I have quite a lot of motivations that have kept me studying Japanese since 2022.
The first thing is, I have realized how far I am getting. For example, I can already read and understand so much and I want to keep improving this because I know that if I continue, I can get advanced in Japanese and that's what I want to achieve.
Another big motivation is that I plan to study abroad in Japan in a few years. It would be so helpful if I could speak (almost) fluently when I'm studying there.
When I listen to my favorite J-pop songs it feels so good to be able to understand a lot. And this also makes me want to continue to understand more and more. And when I'm watching something on the TV for example with friends or family, and someone starts to speak Japanese my mind is like: "Hehe I know what they are saying😎".
I've also been using Busuu for a while and when I do sentence writing lessons, I'm often able to write more complex sentences with only few or sometimes even with zero mistakes. This motivates me a lot.
Maybe one of my motivations can help find your motivations. It's not always easy to stay motivated especially if you think that you'll never become fluent. All you can do is ignore this fear and say: "No! I will be fluent no matter what!". Because you can. You can become fluent. You just need the right motivation and goal. Goals are also important. This can also help you stay motivated. A goal can be to understand your favorite anime without subtitles for example (Yes I know you see this example everywhere xD). But if you aim for a goal like this and say: "If I continue to study, I will progress more and more and I'll be able to achieve my goal someday!", then this can probably be a big motivation. It's also important to have fun studying Japanese. If you force yourself and try to be just disciplined without enjoying Japanese, you won't come far at all. Discipline can only get you far if you are also motivated. That's at least what I think.
My personal motivation is all the friends I've made on renshuu. Goals and positivity are nice, but without people to support you in your goals it can be really hard to keep going. Also friending a few nice people can be good competitive motivation. I find great satisfaction in answering more questions than the friends I've made.
I like to encourage myself to keep studying by reminding myself how little Japanese I used to know. For example, when I first started learning, I didn't know how to read hiragana, but now I get confused when my mom can't remember how to read them! I also remind myself how hard Japanese is to learn compared with learning a Romance language. When I get discouraged, I enjoy thinking about all the times I recognized a Japanese word in anime, or was able to understand a sentence. I think that when I'm about to lose motivation, a good thing to do is study, no matter how hard it feels or if it seems like it won't amount to a lot in the end. Daily studying is the thing that really leads up to fluency...as long as it still stays fun to learn and isn't a chore.
My motivation is to be able to read and understand them, most of the media doesn't included furigana which have both kanji and kana. Anytime I read a kanji that I just learned or currently studying making me motivated and once in a while try to read japanese media with/without english translation and compared my understanding with it. Anytimes it come close to the translation the more i feel motivated to learn. At first its just general meaning as long as the main point/general idea is similar or the same, however nowadays trying to translated it as close as possbile with the translation and retain the actual meaning (since most translation lost a few words).
My main motivation (apart from the fact that I just like learning languages) has always been to go to Japan. Whether it's for a vacation, to study abroad or even more to Japan, being able to communicate in the native language at least a little is an awesome goal to have and having such a clear goal helps a lot with keeping at it.
Whenever I don't feel like studying ir get frustrated by my slow progress I watch videos on Japan or subbed anime or plan my vacation. Looking at what I wanna see in Japan, what I like about Japan and why I wanna go there always helps motivate me.
I think that when I'm about to lose motivation, a good thing to do is study, no matter how hard it feels or if it seems like it won't amount to a lot in the end. Daily studying is the thing that really leads up to fluency...as long as it still stays fun to learn and isn't a chore. 
