Situation: I had once attempted and am considering returning to self-learning Czech. My native language is Polish. The two languages are quite similar, to the extent that there are significant parts of Czech, particularly written, that I can either understand or deduce the meaning of purely based on my knowledge of Polish.
Problem: The close similarity between the languages makes learning much easier. However, this also causes certain problems. The many similarities mean it is easy to assume I already know a given word or grammatical structure. As a simple example, "shopping" in Czech is "nákupy", quite close to Polish "zakupy". When I encounter it in a Czech text, the meaning automatically "falls into place" in my mind, leaving no motivation to internalize and practice the (minor) differences between the languages.
Question: What are the techniques to overcome this difficulty?
(Note that this question refers to interference from native language specifically, so it is diffeent from other related questions such as this one)