It is possible that the exclusive focus on learning vocabulary is the cause of the demotivation.
First, the brain needs novelty and variety in order to prevent boredom.
Second, it is difficult to stay motivated if you are learning something without an opportunity to apply it and thereby seeing its benefit.
What you need to do is put aside the vocabulary learning for a while and spend some on other things. One thing you can do is extensive reading, which is also known as free reading or reading for pleasure. For this, you will need texts in which you understand roughly 90% of the words; you should then be able infer the meaning of the other words from the context. If you find that you need to use a dictionary for several words on a page, the reading of the text is probably a bit too high. This question about graded readers for German gives a few suggestions.
If you are looking for more authentic materials, you could try Bertol Brecht's Geschichten vom Herrn Keuner and his Kalendergeschichten. You can also try these German Short Stories For Beginners and use the tips from 10 Ways To Find Simple German Texts and Stories For Beginners Online.
There are also good resources for reading and listening of the website of Deutsche Welle.
It is a good idea to use the Pomodoro Technique for studying and that you learn words in the context of sentences instead of just translations. What you can also do is shortening the sessions to 25 or 20 minutes and taking a longer break after 2 sessions instead of 4.