I think it's simply bc we both think in English and translate to polish in our minds.
That is probably the biggest mistake people do when trying to speak in a foreign language. Never ever
try to translate from your native language when speaking. One should conceive utterances directly in the foreign language without any help from the mother tongue.
For example, when speaking or writing in English, I myself forget about my Polish completely. Of course, I do make mistakes, but thinking in Polish first, then translating into English would have brought even more mistakes on me. The mistakes I make may arise from improper usage or shortages in vocabulary (or grammar) or whatever, but resorting to Polish would not help.
It is a matter of training to use the language correctly. The best training is, of course, being immersed in a foreign environment. But even without that, you should constantly train yourself in thinking in a foreign language with as little or no help from your mother tongue as possible. This goal is achievable as my own case proves, the case of a man who has been living for many many years immersed in a non-English language speaking environment and having almost none oral contact with English