Mother tongue is like a reflex
Just like your maiden name is always your name, even if you marry, your mother tongue will always be your first language, even if you live in a place speaking another language. I spend a good proportion of my day surrounded in English, but I still dream in French. My husband is perfectly fluent in French, but will ask me for water in the middle of the night in Arabic.
Spanglish is another good example to me. Native Spanish speakers living in an English speaking country have developed an in-between way of speaking. Here is a Spanglish 101 if you are not familiar with it: Guide to Spanglish
Obviously, the same tends to happen in every language. I tend to incorporate some English to my French and some French to my English. Even if you speak both these languages, understanding young French Canadian people is a challenge.
Learning a new language
Learning a new language requires efforts and time, but above everything else it requires you to learn to see the world differently. You will need to shift your paradigm slightly to understand grammar and tenses. It took me a while to understand the difference between “It has rained”, “It rained”, “It rains”, “It is raining”, “It has been raining” and to know which one to use according to what I want to say.
It is easier to learn a language as a child, partly because you are a better learner, but also because of school. Teachers make sure that you practice and take some time for that new language. As a full time working adult, I find it very difficult to spare the time to work on my Arabic. Once your level is good enough, improving becomes easier, as you can integrate it in your daily life, by reading or watching the television for instance.