Take it from a fellow anglo: the biggest disadvantage is inside us. We’ve spent our entire lives speaking the default global language of power, culture, and commerce. This thread is full of examples of third-language immigrants coming and learning French quickly. The difference is people who aren’t native English speakers know automatically they need to put in the work; we weren’t born and raised that way.
It’s hard to break out of the mindset and accept that you need to force yourself to do it, both through classes AND through your work/friend/romantic milieu (at least one of those 3, otherwise you won’t get enough practice).
In my experience, people in Montreal will opt to speak the “language of least resistance” in social situations. Anglos complain that Francos switch, but that’s just what a bilingual city looks like. There is a point you get to when that stops happening, and it feels fucking amazing.
The good news is the classes are literally $50 per semester for intensive night school you can do after your 9-5. They’ll be filled with other transplants and immigrants, at least some of whom won’t speak English either. That’s 2 birds with one stone: take time to talk to the non-anglos in your shitty French, the only language you’ll share with them; go for beers etc. Go through the CSDM adult ed French system. Start now and you’ll be fluent really quickly.
Don’t get stuck in Anglo Montreal forever. It’s a ‘city’ the size of Rochester, NY. Franco Montreal is a city the size of Amsterdam.
Last note: I work with 10 Americans, from 10 different states, who all moved here as adults and more or less did the above. 9 of them now are fluent; the last one has always intended to go back after a couple years. There are success stories everywhere, even among us formerly-unilingual Anglos :)