Some of my generic tricks for language learning are:
-don't just lock in on one learning app or method but always be open to find better tools. Every language has different options. But don't waste too much time on that either - that's the language equivalent of trying every JS framework before starting your personal website
-identify what the hard and the easy bits of the language are. Ask yourself which of those you absolutely need in order to progress and which can wait.
Then tackle the low hanging fruit while you start nibbling at the hard-but-necessary ones and leave the hard-but-less-necessary ones for later - by which time they will be easier because you'll be more familiar with them.
Your perception of what the hard and the easy parts of the language are will evolve with time, stay open to that.
-adding to the last point, identify your strengths and weaknesses towards the language, this includes understanding what advantages your current stock of languages give you. Like, how Japanese pronunciation is easy for a Spanish speaker, but vocabulary acquisition isn't. Or how if you have very good memory but poor social skills you'll be good at building your vocabulary but speaking will require extra effort.
It is hard when they are such close languages! But maintaining is always hard. I compare it to building with clay while it's mildly raining. You need to keep re building chunks that fall off what you've built. Eventually some chunks dry enough that they are solid forever and you use them as a base to keep going.
I don't think you need to put a lot of time and effort in maintenance but you do have to put some with certain regularity
I thought that was a pretty common thing to feel? I mean, the different personality thing. I had never thought about things not feeling so real... I'm gonna definitely meditate on that one for a while
I also got sad when i started replacing my native language with English in my thoughts, and pushed myself to think in Spanish again, have more Spanish speakers around me etc. since i no longer live where i learned my native language, i speak a somewhat "foreign" to myself version of it, which introduced a new type of sadness - that of not being recognised as a local in your own home, losing a specific kind of personality that's bound to a specific place- but that's one I've learned to live with
Been learning Japanese for...uhmmm...30 years? Not at a consistently intense level, but i started at age 12 with a book my grandpa got me for my birthday when i was a full on anime nerd.
I am fluent in a handful of languages so i know i have it in me but, Japanese, oh boy, it's a tough nut to crack. I am conversationally fluent but I'm aware I must sound like a monkey half the time.
Japanese is deceitful because the basic grammar rules are pretty simple once you get your head around them, not a lot of tenses or conjugations to learn, not even plurals or gender for the most part.
But something about how simple it is makes it actually harder to understand, like the meaning is all these nuanced expressions that take forever to learn, and then of course the completely alien vocabulary, which because of the lack of phonetic variety there's lots and lots of ambiguity, too many similar words. And then there's all the speaking registers as used by different classes of people...