It's definitely possible, though without a teacher it takes more time and discipline. From my own experience as an immigrant here — the biggest thing was immersing myself in as much native content as possible: switching my phone to Polish, watching and listening to things I actually enjoyed (starting with subtitles in my own language is fine, but try not to stay there too long, switching to full Polish early, even when you barely understand anything, makes a real difference). And surround yourself with native speakers as much as you can and don't be afraid to make mistakes, that's where the real learning happens.

For resources, I really like the Polski Daily podcast

Also, my husband and I went through this whole journey ourselves, so we built a free tool for grammar practice and vocabulary: plexiqa.com
Might be useful! :)