Motivation is not something you have
The first few weeks run on novelty. Then the conjugations arrive, and the listening does not improve on the schedule you assumed it would, and one evening you skip a session for a perfectly good reason. That is usually the beginning of the end, and it has nothing to do with aptitude.
People who keep going are not more disciplined. They have arranged things so that less discipline is required.
Attach it to something you already do
Most days you do not skip studying because you decided not to. You skip it because deciding when to study was itself the task, and you were tired. James Clear's habit stacking removes the decision by bolting the new habit onto an existing one.
The shape is always the same: after I do X, I do Y.
- After I pour the coffee, I read one story.
- After I sit down on the train, five minutes of listening.
- After I brush my teeth, three conjugations. Three, not thirty.
Give it a few weeks and the coffee starts doing the work for you.
Stop being a student of Portuguese
There is a real difference between "I am trying to learn Portuguese" and "I read in Portuguese". The first frames the language as homework, something outside you that has to be got through. The second makes it a thing you do.
Culture is what moves you from one to the other. Put on fado properly, sitting down, not as background. Amália singing about saudade is not a vocabulary exercise; it is several centuries of a small country watching the sea and expecting people not to come back. Once the language carries something you actually want, you stop having to push yourself towards it.