Arnaud wrote:- Secondly, if you don't know certains words, it's normal that the brain try to replace them by known words, and as french is full of homophones it's particularly easy to do: you could have written: nous devons aller à l'avant. The fact that you hear "à" instead of "de" is probably normal: if you don't know the fixed expression "aller de l'avant", you can interpret it as "aller à l'avant" that is more frequent ("aller à" is super frequent). In general, when I correct transcripts on Italki, I see that people make a lot of mistakes with prepositions: either they simply don't hear them, either they "interpret" them badly.
I think this is the key idea here.
Many comprehension problems come from weak vocabulary, or weak knowledge of the structures we are likely to encounter. What I found immensely helpful, for my French listening comprehension years ago, was not only quite good theoretical knowledge of grammar and vocab. But also reading. I started with lots of extensive reading earlier than with lots of extensive listening. That way, I was getting used to seeing the patterns and words, and the sound was later drawing from this huge pool of "stuff that is normal and could be encountered".
So, I would recommend a lot of reading. And also work on vocab and grammar. Especially grammar, looking at the examples. It helps the comprehension to know, that some stuff does and some stuff doesn't fit in the context. The good old grammar exercises may be useful, but I would definitely say reading will be great, for both passive skills.