I didn't have to worry about this problem before; since I was totally fluent in English i was really confident that I wouldn't have troubles balancing between English and French. But now that I've studied French for a year, I have gotten less confident with English as time progressed. I have the habit of speaking to myself in foreign languages whenever I have time, and for the last month I switched to speaking to myself in French from English. But to my big surprise, my English level plummeted in only 1 month. I no longer feel as comfortable speaking English as before and I sometimes forget how to use some tenses and add an "s" to verbs for the third person singular, which used to come completely naturally for me. In contrast, I started feeling natural speaking French to myself. To recover my English skills, I started talking and thinking in English once again, but while my English hasn't gotten better at all, I forgot how to say so many words in French! Right now I'm getting so bad at both languages and I'm so frightened... How do people ever manage to balance between their languages?
I'll keep in touch with this topic
I've read before that this can be a problem for polyglots and people who feel that they have learned a language. Remember, just because you learned a language doesn't mean you will always know it. You have to work to maintain a language that isn't your native tongue, otherwise your level will decrease over time.