It's just too weird for me that Japan wouldn't have a proper protocol of treatment in this day and age... But I get it, it's for the plot.
I also suffered a huge accident when I was 14 and lost part of my memories, the worst part was that I practically became Dori of Finding Nemo afterwards, so I don't remember 2/3 years before the accident and a year after is just a few pieces here and there. My capacity of retaining new memories was never the same.
I had a lot trouble at school because all the knowledge was also lost and I had to learn everything again. Writing was the most difficult to pic it up because I lost the years we start to learn to write essays and the like, I was a highschool student writing like a kid that just started middle school, I felt humiliated when I read stuff I've written before the accident and I could never write like that again, it was so frustrating that I stopped and it became the most difficult thing to do.
Not remembering important things is TERRIFYING, specially when people come to talk to you and you don't know them or when they tell you things you did together and you don't know if you're actually remembering of it's just your brain creating memories based of others testimony...
It's HARD, so I hate the fact that ML lied about their lives together, but I also understand he did it because he was afraid to lose him, only someone who actually lost their memories would understand how terrifying it is.