Progesterone after letrozole and missed trigger

May

So I did letrozole, had a ultrasound day 1el3 and follicles were not big enough yet so I was told not to trigger and come back 3 days later day 16. Came back and ended up ovulating sometime in those 3 days so we missed the trigger. Now I'm suppose to take progesterone vaginally starting today day 18. All of this is confusing and I dont even know what the progesterone is for or if we missed this month all together. Help????