I can sympathize with you; that wonderful frustrated feeling... no thank you!
Scientific community's thoughts
With regards to language-mixing in children simultaneously learning two languages:
"Language mixing refers to the young child's mixing of both languages within the same utterance before the child is really aware of having two languages in its environment." (1987:27) (emphasis in original)
This definition of language mixing makes it clear that the mixing occurs among children during the time before they differentiate and separate their two languages. The mixing is unconscious and is used by the child without regard of their interlocutor's understanding of both languages (Arnberg, 1987). The children are simply using words that they have acquired to communicate their needs at the given moment.
Obviously, an adult's mind learns languages much differently from a child's, however, this passage does indicate that once the brain has a reasonable grip on the language(s), it will begin to differentiate between languages more effectively.
And with regards to adults already reasonably familiar with two languages:
The term language mixing is also used in reference to adult bilinguals. However, in this reference, the definition is entirely different. When used to explain the speech phenomenon of specific adults, language mixing is a conscious use of a blend of two languages where interlocutors understand both languages.
"Adult bilinguals" referring to those who learned another language as L2, it seems that unconscious/involuntary language mixing is not an issue; rather it is a conscious effort, however, the parents' fluency level was not explicitly addressed, so take this note with a grain of salt.
Another study mentioning the ease at which children overcome unintentional language mixing: http://www.concordia.ca/cunews/main/stories/2013/01/15/growing-up-bilingual.html
Another model relevant for the study under consideration was proposed by Myers-Scotton (1993:75), known as the Markedness Model, in which he notes that a bilingual individual has a sense of markedness (1993:75), in regard to the relationship with the interlocutor who essentially the one choosing the code in the conversation. In such situation, the speaker is perceived as a rational actor who can make either the unmarked choice, the more secure and the more expected choice, often used by the speakers, or the marked choice which is generally unexpected in interaction (Myers-Scotton 1993:75). Nevertheless, it is essential to mention at this point that the concept of the social importance of language choice should be applied with a dose of caution to the speech of children in general as they do not play the same role in society as adult speakers.
generally speakers are aware of the effect of their switch
From this document, citing
Myers-Scotton, C. 1993b. Social motivations for codeswitching. Evidence from Africa. Oxford: Clarendon Press.
This indicates that adult bilinguals are conscious of their language mixing.
One of the greatest ways to familiarize yourself with your target language is to speak it (and eventually think it).
From this not-a-scientific-study:
I won’t ever get tired of repeating the golden rule of my language learning advice: speak, speak, speak!
Speaking your target language will make your brain more used to it, and eventually you should be able to go into "Polish mode" or "Spanish mode" or "XYZ mode". I can attest to this myself.
And another not-a-scientific-study:
When learning a new language, you'll start learning words from your first language and translat-ing them into the new one. After a time, you should adopt a strategy where you are thinking in the new language and no longer translating the language in this manner. If you are going to avoid mixing up your languages, you need to start thinking in the new language.
Being able to think in your target language will enforce the "XYZ mode" ever so much.
You're aware of the problem-- that's halfway to solving it.
Until your brain adjusts to having another language around, those mix-ups may not go away for a while. However, don't lose hope! It took me a little while to find actual scientific studies that I felt I could use: take that as an indication that it's not too big of a problem; it should go away once you achieve more familiarity with your target language.
Familiarize yourself more with your target language and it won't be a problem for too much longer.