How languages influence each other

I really like to study a new language. To me it is the first step to know a country and its culture. However it is not easy to master so many languages.
My trip to France next month encourages me to learn some French first, which I gave up long time ago, not only because French is hard to learn, but also because it does have some influences on English as well and the first obstacle is pronounciation. Of course, a new pronounciation system is basic when learning a new language, but I have been headed into English for a long time and sometimes I am confused about it. I just afraid that I may lost the sense of English as soon as I start to learn French wholeheartedly. This situation reminds me of the time when I had to choose one language to learn between Japanese and Korean.
I learned Korean myself when I was in high school and I chose Japanese as my major when I was in collage. The two languages are so similiar in some way, but on the other hand they do have a lot of differences. I like both of them but I know i can't speak them at the same time. I have to forget all the Korean words when I speak Japanese and of course when I watched a Korean drama, I could never speak Japanese fluently - only when I started to watch Japanese drama and adjust my head to a "Japanese head". It sounds silly, but I feel that there are two switches in my head and I can't turn on both of them. That's why during my four years in college I never watched any drama about Korean except to listen to Korean music.
Going back to English and French. I haven't felt such strong influences of them compared with Japanese and Korean. Maybe it is because I'm just a beginner of French. My dear friends, is there anyone who has the same feeling like me when learning languages?

[Good article.  Yes, I always feel that way learning a new language.  Children younger than 12 years old seem to be able to learn new languages fast, but older people have a much harder time of it.]