Since I left France the only way to maintain motivation is continue reading French books (OMG there is so much to chose from). But bittersweet having to make a choice as I now want to dive into Italian. And so with every hour I spend on Italian I see my previous skill and speed with which I could form a French sentence wane ...
The best thing about language is the vast ocean of superb material to chose from as you dive in. It's like a honeymoon phase with the country and its people.
Spending a whole year learning and then let yourself get rusty is almost like spending 12 months on crafting a magic key then throwing it away before entering the castle. Not that there is anything wrong with "learning for learning sake" but I know the feeling when you can literally see it slipping from your grasp and still can't stop it.
French Spanish English maybe not so much because it's everywhere so practicing it is easy. But many languages you have to actually be there if you want any chance at all of being good at it. I liked being able to brag about what rare, exotic languages (for a European) I was able to converse fluently in. After leaving these countries, breaking up with girlfriends, change of social circles, etc I felt like a fraud every time somebody introduced me with "and he is fluent in <xyz> can you believe it" ... the conversation had to quickly be corrected by "oh I'm totally rusty" which got the response of "oh don't be modest I am sure bla bla" which made me regret that the whole thing ever came up. Putting much effort into languages is quite a humbling experience.