What do you do when you have a student who just doesn't understand a concept and is getting upset with themselves?

I teach an ELL class and have many different grade levels. While the curriculum is tailored to their grade and skill level, I can see how they compare themselves to their English speaking classmates and get frustrated. I've tried to reassure them they are learning a new language that requires extra time so they need to be patient with themselves. I also try to remind them how far they have come so far. Any other words of wisdom I can share with them?