I want to offer a slightly different view and advice to what others have said in this thread, although it is similar in tone. I also am someone who flits between things and has often felt they don’t challenge themselves or stick with things. I think a lot of the rhetoric about discipline and productivity talk about habits is all well and good but it has never worked for me and, personally, it just makes things worse. I think you are probably similar to me, OP, in that forcing yourself to be “disciplined” or “building a habit” won’t work and you will end up feeling worse about your situation.
So, my advice regarding motivation is twofold. First, just focus on what you enjoy doing so long as it is using the language. Focus on what you love about the language - this can change from month to month or even day to day and that is okay. The more time you spend with Japanese the more it will open up to you and it will become easier and easier to study/consume the language. Of course you may push yourself sometimes but only if it is something you want to be doing. Don’t overload yourself - if you have very high motivation one month then don’t throw it all into SRS. Keep the SRS easy and simple and throw that motivation into studying from a textbook or videos or start looking at native content, etc.
Second, all this talk of habit formation is all well and good but honestly I think for some people (like myself, probably you) it isn’t that helpful. I now have a rough habit of studying and using Japanese but it is an accident of circumstance and not something I worked on. I don’t believe in streaks and personally find the idea of doing SRS daily to be insane. Just show up more days than you don’t and you will be fine. Using the language is far far more important (at least once you have the basics down). My own progress has been relatively fast given I work full-time, have a social life and other hobbies, etc. In real life many people I know look at my study habits and say like “wow you must work so hard” or that it is discipline etc but it is none of that and it would be self-aggrandising for me to agree with them. The truth is that I like Japanese and I like language learning so I want to show up to study and use Japanese. Not every day but most days. This love for Japanese was something that grew as I studied more and not something I had at the start by any means.
One very very important thing that I think isn’t mentioned in this thread (I skimmed it so sorry if I am repeating something) is that the beginning is by far the hardest part. Later on it is tough because progress feels extremely slow however at the start things feel impossible and nothing really makes sense. Just keep going and it gets much more comfortable - it won’t feel like that forever. There are also a lot of meta-learning skills that you have to learn at the start and they take up a lot of mental energy.
A final note, concerning moving to Japan and learning Japanese: You refer to Japanese as a hobby but if you are serious about working in Japanese and getting good in a short period of time (3 years is a short period of time, in this case) then you need to not think of Japanese as a hobby and start thinking of it as a lifestyle. Alternatively, you can look for English speaking jobs in Japan and lower your Japanese learning goal to N2ish in 3 years or something (not fast but also not slow for a hobbyist) and treat Japanese as a hobby with less pressure. Once you are in Japan you will be forced to take Japanese seriously by circumstance anyway. I will also say that the vast vast majority of learning happens after N1 (I am not there yet but I am studying for N1 now); language learning is lifelong and not something you can “complete”. Many online learners are hobbyists or do not speak the language even if they can understand it so a lot of online discussion is not practical or won’t match with your experience if you come and live here. Don’t compare yourself and just focus on having as many meaningful connections with the language as you can and you will be fine. Just keep going!