I used to speak fluent French (forgotten a lot of it as it's been several years since I lived there) and now speak decent, but not perfect Spanish. Unless you have a natural talent for languages, which I definitely don't, then you can't beat immersion. Having a Spanish gf probably won't help at all, it'll actually be a hinderance. When I arrived in the country, I never put any effort into any situation that required much more than "buenos dias" as I'd just call my Spanish-speaking wife and get her to sort it out for me, so I didn't really learn anything. She also had no patience in terms of teaching me as she speaks four languages, loves learning in general and couldn't understand that what was easy for her wasn't so easy for me.
What did help me was when we sold our house and moved in with her parents for a few months as they didn't speak a word of English, so I had to try to communicate with them when she wasn't there. I've never taken a proper Spanish class, but I used to look at stuff for sale on Facebook marketplace etc and car videos on Instagram etc so I picked up a few words from that. There's also loads of free resources on youtube I've never personally used but look really useful.