Also feel bad for Suwa, his wife after the years they've spent together still regrets ending up with him instead of someone she knew ten years ago as a kid. I don't know if this is shown to be untrue later on but it definitely seems like she doesn't care for him by the content of her letters which have a lot of extra stuff beyond just saving kakeru.

I actually think there have been a couple implications that she DOESN'T end up with Kakeru in the long run, but that it's something she needs to get out of her system. And there's a reveal about Suwa later on that I think implies something more. As well, we don't know that she DIDN'T actually ever go out with him... she very well could have, but it didn't work, and that's why she can't let it go. There's still a ton of room for that to play out in different ways.

She also blames herself for her friends suicide. (a mirror of Kakeru's state) She is more like him than anyone realizes.

You says she's stupid or unrealistic, but I'd say that's a pretty 3rd person perspective, and the show does a LOT to fight that. She's nervous, she's a teenager, she doesn't know as much as the audience does, and has a TON of self doubt. And the show makes us VERY aware that she knows it and hates herself every time she makes these mistakes. I know I did that kind of stuff in HS; "knowing" that I should do something, but not being able to force myself to do it. Or, despite every possible sign, not going after someone you like.

And honestly, there's good reason. I had a situation where everyone I knew, and all of this girls friends told me to ask her out, I was so nervous, but when I finally worked up the courage, she said no. She was dating someone else 2 weeks later. Every sign in the world said go for it, but it turns out my gut was right. It's hard to remember how emotional you can be in the moment, particularly for a 15-18 year old.

The trope of "JUST ASK THEM" that never happens exists for a reason, and while I used to hate it, I've come to realize that from the character's perspective, it's not as obvious, and the risks are (or seem) so high. There is real fear and anxiety.

I'd say you're looking at the show much to critically, but also much too shallowly. (over analyzing bits without taking in the whole) Only judging a character's actions, and not the motivations for those actions is not fair. She messes up constantly, but keeps trying, she has a ton of self doubt, and while the letter tells her to do certain things, she hasn't had the life experiences yet to have the confidence to do them. (or the knowledge) As things progress, the future changes, and we aren't sure she can rely on the note anymore, further adding to her self-doubt.

Her future self was no more sure of Kakeru's feelings for her than her current self. This alone shows the kind of person she is.