Hello Diana I understand you very well, I am not studying modern languages like you. I just enjoy learning languages. I feel the same sometimes because there are a few moments when I am speaking in english and some words in french pass through my mind, and when I am speaking french it happens more frecuently specially when I run out of words in french to express what I want to say. I have read something about thinking in a language and there is a technique called shadowing, this is the link: http://stujay.com/2015/10/20/how-to-develop-thought-fluency-in-a-foreign-language/. I hope it helps, also I have read from many polyglots that it is better to master one language at a time specially if both are related.