I was wrong with the height, it's like 40 inches tall now.

Yes, you can cut it back now. 'Etoile Violette' is a pruning group 3 clematis, meaning it blooms on new growth and should be pruned hard every spring.

Check on the Clematis forumf or specifics on growing and caring for clematis.

so the 40 inches that are out now, when I cut it, won't stress it?

Nope! Clematis vines at wholesale growers are cut back routinely during the growing season to keep the vines compact and untangled. With very few exception, it never hurts to cut back hard on a clematis.