This is my recommendation for getting overwhelm under control, and preventing burn-out:
Slow right down on the new Lessons. In fact, stop doing any new lessons for a while, until the feelings of overwhelm lessen and begin to fade away.
Continue doing reviews every day. If you can do all your reviews every day, that’s good. But if even doing all of them every day is too much, allow yourself to slow down on reviews, BUT … always do at least one review session every day, so as to keep your daily reviews routine going. That’s probably the most important thing, to keep that routine going.
As you continue to do reviews every day, while pausing new lessons, you will steadily make progress on your overall review schedule, and it will eventually get back down to a manageable level, so that you no longer feel overwhelmed.
Only when the number of reviews gets back under control, then you can finally start doing new Lessons again. By adjusting how many Lessons you do, over the long run, that’s the best way to manage overwhelm and avoiding burnout, IMHO.
Since you already have life-time subscription, you might be okay with focusing less on ‘speed’ and ‘efficiency’ and more on ‘slow and steady’ and ‘enjoyment’ of learning Japanese (in particular with SRS). If so, then you might want to check out the 
Let’s Durtle the Scenic Route 
thread-group (informal), which is all about doing just that!