I am not answering this from any position of knowledge, just from my own experience. I started intensive practice in the Mahasi system back in 1980, and have had many ups and downs. The downs have been like you described, even going into periods of binge drinking. One of my teachers here in the USA said something that helped me the most. It's not going to sound profound, but it hit me as the most profound insight, and helped me be OK with the downs. He said it was all part of the process of purification, nothing more, and nothing to get bummed about.
So I just placed my rear on the cushion, even though it was the LAST thing I wanted to do. And then started the walking. I am not going to say anything profound happened. Let's say I got OK with not feeling OK. But since that comment from this teacher, I have just done my level best to keep practicing.
I still have down periods, and I still have a drink now and then, but I just re-take the precepts and start again. For me, that's the magic, you can always start again..until you can't that is. But I have developed a taste for sitting with existential malaise, and it's quite favorable, paraphrasing Churchill. But it is definitely and acquired taste! But I will say this, meditation has been such a relief when I stick out the down times and just be mindful of disgust, boredom, etc.
Best to you my friend.