There is a loooot of great advice in here, and I’m just going to throw in my own two cents because your initial comment sounds extremely similar to me about 2 years ago.
I have more free time than most, but I could never force myself to study consistently, whether it was due to quick burn out, or just another sudden hobby I had picked up that I couldn’t stop thinking about.
I struggled with guilt, and how slowly I felt I was going, especially when reading how much faster ‘everyone’ else was going.
After a really low patch, I decided to go to a psychiatrist, and they diagnosed me with ADHD and a few other fun things for the collection.
I was personally prescribed with something that worked really well, and it was an instant change in my ability to concentrate. Even with the help of that, I still have to work on my own willpower.
Now, instead of having to go through what feels like a locked door without a key, I have a keychain with about 5 different keys to sort through. So basically, I only have to convince myself to find the right key, instead of hopelessly wondering how to get out of the room.
After all of that, the next part was not comparing my progress to the progress of other people.
I still do get envious when I see people learning things faster, and with a higher retention rate, but I don’t let that feeling overwhelm me. I acknowledge it, and then move on.
Instead, I try to think about all of the little milestones I’ve made. Like being able to recognize and understand a word, without having to think about what it’s meaning is, or when I see something go up to Master on Wanikani, or Seasoned on BunPro, or even that one time I remembered how to write a kanji off of the top of my head.
Like yeah, I’m not as good as other people, but I’m better than I was, and that’s all that matters.
Like others have said, it might be worth just finding out if you do have ADHD or something similar, and if you do, your psychiatrist can help you down the right path in how to deal with particular things attaining to said diagnosis. And if you don’t, it’s good to have someone like a psychiatrist to help you talk through how you’re feeling.
The good thing now, is that there are so many different communities online that you can find yourself fitting in with very well, which could help you find different ways to deal with certain things you’re feeling.
Habits don’t always form easily, and they can be much easier to break. But don’t beat yourself up. You can always try again. Make your goals realistic, and start off easy.
If you don’t think you can bring yourself to do all of your reviews, just open up BunPro or whatever you’re using, and do 1 or 2 reviews. That can sometimes get the ball rolling, and before you know it, you’ve finished all of the reviews you had.
The good thing about living in Japan as a foreigner, is that the vast majority of Japanese people will assume you speak no Japanese, so even if you say something as simple as 大丈夫 (だいじょうぶ) you’re going to impress them.
Another thing I would recommend if you’re planning to come to Japan, once you’re getting on top of your mental health (which is the most important thing), is looking into different websites for talking lessons.
I put those off for so long, thinking, “Oh, if I just pass N4 first, then I will be able to speak better.” but unfortunately, it really doesn’t make a difference how many kanji you know, or how much grammar you know.
I very recently started taking lessons and just practice conversation in Japanese, and it made me realise I’m not that much better at it than I was a year ago.
Reading, talking, writing and listening are all independent skills, and while they do support each other, you still need to work on each one individually.
Since starting those speaking lessons, they are helping me solidify the grammar I do know, or helping me understand how to use certain words in context, or why you should use one word over another if it seems like they have the same meaning.
Sorry for info dump, I tend to ramble, especially when I feel like I can relate to how someone is feeling 
It’s nice to see you’re already taking little steps and feeling better about it.
Good luck!!
