I hear you. It does get a bit overwhelming once those numbers get big. I just waited until I had time, and then hacked away at them a hundred or two at a time. I found it important to not worry too much if I got them right or not. (I usually take time to think about a word if I can’t remember it, but with a pile that big, it was more productive to just plough through.)
After each review session, I’d look at the page that shows which ones I got wrong, and try to remember them for next time. That helped a lot.
But really, it’s about time and motivation. As with anything else, once you stop a habitual activity, it can be hard to start it again. So look within and see what’s motivating you. In my case, I want to finish this damn thing early next year and get on with my life. But everyone is different.
I’d suggest (if you want any suggestions) to try to get through that pile as fast or slowly as you need to, without doing any new lessons. You can divide it up and say that of the 1400, you’ll do 100 a day and finish it in roughly 2 weeks, for example.
And then once you get it down to zero, before any new lessons, really think about what pace works best for you, and try to stick to that pace, even if it’s only 5 lessons per day.
Or alternately, you could just give it up for a while and come back when you’re feeling properly motivated. These things (motivations) come and go. Sometimes it’s not worth trying to force yourself into doing something that you don’t really want to do right now. Look within and ask yourself how you really feel. If you need more encouragement, I will say this: you’re almost halfway there.