It actually gets harder in terms of time available, because the more words you add the more time you'll need to review old ones, while when you're starting you can use that time to learn new words.
If you struggle with one word, then you're doing it wrong. Also, repeating it all day is bad because I think you need to let it rest for a while, so to speak. Try this for example: take at least 7 words and try to say all them in random order, while being conscious of their meaning while you say them (like, in English if you say apple you know what you're talking about even without picturing anything or translating to anything, same thing). Then after 5 minutes, 10 and 20 minutes check if you still know them. If you remember them after 20 minutes you should beable to remember them till thenext day.
Of course once it's in your long term memory you need to review them, it's not like you can learn things for good.
Short term memory can hold only few elements, so if you do 7+ then they can't probably fit there, that's why I told you at least 7. If you review the same word over and over it doesn't work imho.