For me, the key is loosely following a programme.
I discovered I do best with numbered programmes, like RKS, which had workouts 1-30. I do them in order, but not necessarily on their schedule. I'll take a day off or do something else as necessary for my schedule, how my body feels, or just to give myself a break from one instructor. But then I go back to wherever I left off, and keep going until I've done them all.
That gives me the perfect blend of a structure (I know what's next on deck), and the feeling of progression, which is important for my motivation. But I always give myself permission to take a side detour and come back to it.
I'm currently on that kind of pause from Caroline Girvan's EPIC as I irritated my shoulder from too many pushups & overhead snatches. But when I get back, I'll pick up from workout 38 and keep going at whatever pace, until I get to workout 50.
I'm in the best shape of my life.