Just like mastering anything, to become fluent in the language, it takes time. Getting used to the language takes time. Getting rid of mistakes takes time. Being able to read that book takes time. Being able to understand television takes time. Making small talk in the language takes time.
I hope I made my point.
In order to learn the language, especially the first foreign language, it takes time. That is why, the reason for learning have to be strong.
Motivation is the force fueling the language learning process. But it needs the strong source, the reason for existence.
Language learning also have to be fairly consistent. In accordance with the forgetting curve, the more time it passes since the last time the language is used, the more of the language is forgotten. There is also the rule, that the more somebody knows the language, the slower it leaves him or her. But if a person in the process of language learning, than the knowledge could not have been so big.
Language is also a really wide thing. To know the language, a person have to first develop the 4 main skills - reading, writing, listening and speaking. But it is not the end of the road. The person then enquired the cultural knowledge, the word phrases, the rithm, the grammar and all the other things, that makes languages hard, but so beautiful.
But as I mentioned, to finish the journey, there needs to be a lot of motivation. There is always going to be people, trying to accommodate you by speaking english. There are going to always be interesting series on the television. Interesting book to read. You are going to be tired after a long day. The job will be stressful. Then there is that party, you promised to help with. Or doing a favor to your family member.
But if these reasons are big enough, to stop you from practicing the language for at least 15 minutes a day, than you reason for learning language is not strong. The will power alone can not get you from point A to point B on this journey. Reason and motivation are the most important part, since the journey is so long.
There are also some positive habits (like consistency), that can help you with language learning. Or maybe you are more suited for group learning, since it helps with motivation.