How to improve my English skills while using it daily with people who are not proficient in using it, without absorbing their mistakes?

I have some experience with the struggle you are describing in your post.

My boyfriend with whom I am living is not from an English speaking country, nor am I. As I am not yet proficient in his mother tongue, we use English a lot to communicate. During the first months of our relationship, I could realize how I was adopting his mistakes. So I had to take some measures.

1. Firstly, I tried to classify which mistakes had been occurring frequently and to identify them every time they'd appear. (That works only when being concentrated all the time, but it works!).
2. Secondly, I watched and read everything in English (news, movies, series, books etc.). That made the difference for me because I "consumed" at least some "proper" English and had enough "material" to reflect on.

On the other hand, there are quite some advantages of such a situation. Actually, you can use the situation for your own gain. My boyfriend and I are correcting each other's mistakes and learning from that. The change of perspective from native speaker of language X and native speaker of language Y speaking in language Z is interesting because they can explain (e.g.) grammar from a different point of view. (By the way, there are more non-native English speakers than native English speakers so that it will even be a valuable skill to understand different accents in the future rather than to speak perfectly - but I understand that the beauty of a properly spoken language is something to strive for.)