When you're feeling unmotivated, sometimes moving your body mindfully and patiently is a great way to invite more energy back in. Trust me, there have been plenty of times I've wanted to lay in savasana the entire class and was so happy I didn't by the end. Let your breath guide you, and you'll start loving the ease and availability of adding yoga into your routine.
Don't get me wrong, yoga isn't easy, but it is adaptable to you. If you want a vigorous flow and a challenging workout, choose vinyasa or power. If you need to move but you don't want to totally be dripping sweat at the end of the workout, take it down a notch and try a less demanding sequence or use a few beginner poses to build your own.