The more scientifically minded of you may be interested to read this article. Not being particularly of that bent myself, I can still recognise an incontrovertible graph when I see one, and would have to agree – not just because the science says so, but because my gut confirms it – that motivation may be the single most important factor in learning, including language learning.
Notice, by the way, that with any practice there is progress regardless – yes, slower progress when there is low motivation, but progress nonetheless. We all don’t feel it sometimes. The thing I love to do most in the world – writing – I sometimes have to drag myself to kicking and screaming. It’s still worth it, though. Not that I don’t sometimes produce rubbish. But every minute I put in to writing is going towards making me a better writer. And it’s the same with you and language learning – whether or not you are feeling it.
But if your progress is stalling, it might be worth asking yourself how your motivation is doing these days. There are some tips here to help you nurse it back to health.
— for more hints and tips on language learning, buy Conquering Babel: A Practical Guide to Learning a Language here in the US and here in the UK.