The key I found was to avoid self flagellation.

Try your hardest to do each session, but if you miss a session don't try to make it up. Just get on and do a normal session the next time it falls due. You're in it for the long term, so long term it doesn't matter if you were intermittent when building the habit, or the occasional session gets missed for a reason.

This, plus start small. Just do those 5 or 10 minutes of karate exercises per day, at a fixed time, or 5 new flashcards per day.