Get tissues

Please watch the video first before reading my thoughts.

OK so what I think the story is about is that Rin a 17 year old girl is living in a simulation from her mind. She has this tablet that has the power to create any world she wants. Like a futuristic minecraft. But everyday, her messages are empty and shes lonely. So then, on her 18th birthday, she creates a swing and when she went to touch it, she had lots of memories and flashbacks, possibly in the program, when she turned 18, the program wanted her to know the truth, or moreover, her dad did. Then implying that Jupiter or a giant planet was going to crash into earth. So then the dad knew that this was coming so he built a ship that will fit her as a child and as an adult. So when Jupiter almost crashes, she is sent into space and the cords sticking out of her are the nutrients and food in her body. She's in a deep sleep but she's alive and that's why we see her creating those worlds. In the end, we also see that she has a message. Is from her dad, possibly in the program, it's in Japanese in the video but it translates to:
________
To Rin From: Dad

There was far too little time left after you were born. I wonder how much love I was able to show after your mother passed... Seeing you cheerful and well gave your old man courage.

It really would have been better if we could have gone together, but I couldn't make it work. I thought I wanted you to forget everything and go forward...knowing you, you'll be okay. You'll realize your loneliness, and surely remember me.

I believe that you'll grow strong and read this letter. I wish we could have made more memories. I'm sorry. When you were little you probably didn't understand well, but now you should.

I'll say the words from that day once more.
_________

A couple of key points to keep in mind here:

"dore dake no aijou wo sosogeta darou ka..." is not asking if he gave her enough love, it's reflecting "oh, how much love I showered upon you." Corrected, thanks. As you can see in the MV, he doted on her and constantly spent time with her even when busy with working on his invention. In fact, one could perhaps argue that maybe he would have been able to complete a version that could save both had he not been the greatest goddamned dad of all time ever. Edit: Guess his regret is a bit ironic, then, which is why she doesn't hold it against him.

"yuuki tzukerareta" could be loosely translated into "you gave me strength," but I think that considering the nature of his sacrifice here, "courage" is the more accurate sentiment

"zenbu wasurete hoshii" is the one line that gave me the most trouble, as I could see it either way; a) him wanting her to forget him so that she doesn't feel lonely or b) him wanting to forget about his desire to be there for her in order to save her. Grammatically corrected, thanks. I could very easily be was wrong on this one, in which case I would go with "I thought I wanted you to forget everything and go forward...knowing you, you'd be okay / You'd realize your loneliness and remember me." I think that the implication is just that she's so small he doesn't think she'll remember him after very long (EDIT: and thinks maybe it would be better for her to forget him), but has faith that she'll eventually remember from the clues he left her (EDIT: which he leaves her because he decided that actually, it's better for him to leave something behind because someday she would definitely realize her loneliness). Another detail to note: when he puts her in the rocket, and she is first in space, she is CONTINUING to cry streams of tears down both sides of her face in spite of her being unconscious and in the program - so he doesn't force her to forget, it just happens naturally over time.

"you were so young back then" is a bit of a stretch on paraphrasing; he just says "when you were young/small," with no emphasis about her being very young or so young. Just "you were too young back then to understand what I meant" is fine; emphasizing doubly in this case starts taking us a bit far from the original text for my taste, personally.

Cutest bit ever: we don't get to see the closing words, nor do we get to hear them when he speaks to her as she cries in the rocket, but they're powerful words of love, judging by her face after finishing the letter.

so yeah, that's my explanation. You can say I've been watching too much matpat.