I am still struggling with motivation, so I am not the best one to answer this question. However, I think just having a "minimal study session" helps.

Like, imagine your worst ever day: something like a day when things are hectic and you couldn't even gather enough motivation to study. Now, try to make a routine that you can do even on such a day, and then promise yourself that you'd do at least that minimum study session no matter what it takes.

Of course, it's too easy for that "minimum study session" to be the "default study session" so I've got other things that would at least make me do the "default study session" instead.

In my case, the "minimum study session" is just to do one Anki review for each deck I have. That's it. My "default study session" would be "finish all reviews in all Anki decks". I also have an "intensive study session" involving adding new Anki cards to my decks, as well as consuming stuff in my studied languages, but it's been a long time since I did those.

TL;DR: Make it easier on yourself to stick to a routine; have something that you can do even on your worst days.