yeah, i'm so confused too...
He had to learn to love himself, and he's determined to do things differently
As far as I understood it, he didn't hate himself. Only his situation. And sure he didn't want to end up suffering so probably wanted at least his past self to not be gang raped (?) like he was. But guiding him should be enough. So why, how did he fell in love with his younger self? I get it that they are different. His experiences probably changed him quite a lot but this is still surprising. They also should look quite alike (not that it's easy to tell here since every character look alike). 10 years is long but not long enough to drastically change his every feature. Not knowing it's yourself can be understandable (younger one who fell with the older one) but the older one knows. I just don't get it.
Ok, them getting an injury in the same place threw me. It's hard to tell with the translations, but I think he's the oldest brother Eric and that kid is Frederick. The younger brother who slapped him. Fredrick now finds himself in the same situation Eric was in. They did merge, and I think it's Eric who is now in Frederick's body. He promised the devil his heart, and Frederick became his heart since originally Eric said he didn't have his heart. That's what I'm getting from it when I looked it over again
I'm just getting more and more confused honestly. I agree that Eric (older dude) in Frederick's (younger dude) body. However I don't get what you mean with the first part of your reply. Are you saying Frederick is not the older brother but the younger brother and thus there are actually 3 brothers in there? I don't believe the mother looks old enough to have a kid as big as Eric (then again this is manga world but still).
The last story was a bit weird. So he basically fell for his younger self? (and vice versa)
Or did he just love himself and wanted to protect his younger self? Even then how did it evolve into a romantic love? I'm just so confused.