Learning Polish fluently and integrating into the society felt like an impossible task
Do you feel integrated into UK society? According to the BBC the modern UK is simply a place where lots of people with nothing in common live near each other and the only common elements are empty, undefined slogans....
Who do you socialize with?
we did not manage to learn Polish
What kind of efforts did you make? Lots of people put in very... misdirected and inefficient efforts and then when it doesn't work right away they give up. I take it your first language isn't slavic (since most Slavic speakers learn without much effort) so that means Polish would be more difficult but it's very doable though being a couple is a little harder (since you're already each other's primary company).
The simple fact is that any kind of migration is traumatic and will at times lead to feelings of alienation and not fitting in. Lots of things in the media try to portray it as a simple process with no emotional burden but that's a lie. Just recognizing that you've chosen a difficult path that inherently comes with occasional feelings of regret and second thoughts and alienation can be a help in dealing with those feelings when they arise.
The idea of assimilation was a way of dealing with those negative aspects of migration as it gave those moving a long term goal to work for, but no one is supposed to assimilate now because what can you assimilate to in the UK? It's like trying to carry water in a basket.
In Poland at least there are things to assimilate to (or integrate with). It's not easy but ultimately is probably more psychologically healthy.