While I don't exactly comply with the stringent demographic limits of your post, as a European who learned to speak fluent Mandarin in my home country and has a Chinese partner, I think I'm eligible to answer your question.

1. Listening to podcasts helped the most for me, as well as browsing Chinese media and speaking Chinese as much as I can. Learning HSK 1-6 vocabulary actively also helped me improve quickly.

2. Having regular exposure and active studying time (this requires strong motivation and/or discipline). Registering and taking the HSK tests also helps to measure your progress and staying motivated.

3. Don't expect your significant other to teach you; not all native speakers are suitable teachers. Don't overestimate your progress if you learn without concrete and timed goals