It is completely normal to need to scale back your language learning as life, energy, and motivation shift. The best strategy is to be honest with yourself about your current capacity and to rotate languages intentionally: one main focus, one or two maintenance-only languages, and perhaps one new language you explore slowly. This prevents burnout while still making measurable progress.
After five years, you likely have a solid foundation in several languages. The key is to accept that you cannot advance all of them at the same speed. Your approach of keeping Japanese as your primary focus, Portuguese as a leisure maintenance language, and starting Mandarin Chinese as a slow burn is smart and sustainable.
For your main language, set a small daily habit that is truly effortless, like 10 minutes of active study (Anki, grammar review, shadowing) and at least 20 minutes of comprehensible input (reading, listening). Track progress in a simple log to stay motivated. For maintenance languages, aim for exposure that feels like a treat: watch a show you love, listen to a podcast while doing chores, or read a few pages of a book. The goal is not improvement but retention and enjoyment. For a new language, focus on the sounds and basic phrases first. Give yourself permission to go very slowly. Use resources like a good phrasebook app or YouTube videos that explain pronunciation.
Tradeoffs: Prioritizing one language means others will likely plateau or even regress slightly. That is fine. You can always rotate your main focus later. If you feel guilty about not progressing in Portuguese or Mandarin, consciously reframe: you are maintaining a valuable skill that will be easier to revive later. If you try to push all three, you risk abandoning all of them.
Concrete next steps: 1) Write down your current energy level on a scale of 1-10 each week and adjust your daily targets accordingly. 2) Choose one main resource for Japanese (e.g., a graded reader series or a podcast) and commit to using it daily. 3) For Portuguese, schedule one "maintenance session" per week, but let it be flexible. 4) For Mandarin, decide on a maximum of 5 new words per week and just practice tones and simple greetings. 5) After 2-3 months, reassess. If you feel more energy, you can add a little more.
Remember, language learning is a marathon, not a sprint. Five years is an impressive achievement. Listen to your body and your mind.