How can you lose a streak?
I'm sorry if im not using this bit well, but how? Do you only make a day's streak if you get 50xp? Or if you do a single lesson, would that still count?
It depends on what your daily XP target is. You have to meet or exceed that target each day or you will lose your streak.
I have my streak set to 1 XP per day (even though I regularly exceed that figure).