An interesting thing is mixed languages, or languages that combine elements of usually two distinct languages. One of these is called Surzhyk, or a mixture of Russian and Ukrainian. I actually did not learn proper Russian first, but instead Surzhyk, although I thought that it was Russian. So later when I realized that what I was speaking was a mixture of Ukrainian and Russian and some of the words that I was using that I thought were Russian words were in fact Ukrainian, did I start separating the two languages. So I had to make a conscious effort to not use the Ukrainian words when speaking and instead focus on using their Russian equivalents instead. In this way I learned how to speak proper Russian (although still with some Ukrainian influences) and lost much of my Ukrainian.
Wikipedia Article on Surzhyk
Another article on Surzhyk:
Surzhyk: The Sociopolitical Significance of Ukrainian-Russian Mixed Language