tons of practice, and iron discipline.
Sorry but have to disagree completely, on the two main fronts - that loving someone and wanting to speak to them in their language is flimsy motivation, and that Polish is so difficult you need to bury yourself in grammar books for years before being able to hold even a simple conversation.
The best motivation for learning a language is to communicate, and to be understood. When falling in love, or being in love, with someone, all you want to do is communicate with them, in whatever and every way, and this is particularly intoxicating when they speak a different language to you ... there is so much richness, so much potential for so many charming, disarming exchanges. In the first stages of language learning we are like children again, sweetly vulnerable, entirely in the hands of the one who knows. And it is the easiest way to learn a language, with a lover, because neither side tires of each other's words, and both are completely forgiving of the other's mistakes (pretty much ... : ). I think it is absolutely the best way to learn, and the quickest, and the most like natural language acquisition. And anybody can access a language like this - you don't have to be someone who is comfortable with grammar learning, etc.
And now to Polish: is it so hard? I haven't found it so. I have loved the tricky bits. And I think that if it is properly introduced, it isn't hard to pick up - but the problem is, it so rarely is. People see all the grammar tables and endings and just think it is impossible ... of course it isn't. Babies learn it, like any other language. I've watched how children acquire Polish, and heard it, and my own daughter was entirely fluent until four (when she went to English school) - it's just the same as any other language. There is only one right way - only one way sounds right. You have to just open yourself up to that, truly understand what that means, and not come up with crap that isn't what anybody else is saying.
I think the hardest bit to get to grips with, if you haven't learnt, e.g., Latin, is that words change and behave differently to their static English counterparts. (Incidentally, I think English is easy to be bad at, and very hard to be good at ... ). But you can get accustomed to this - your best bet is to listen and listen to people speaking Polish, and just accept that's the way it works.
What can I say ... learn it on the pillow, and don't go into it thinking it's hard - think of it as a rewarding sort of puzzle, and delight in its complexity.
Of course you can do it.