I also suggest Haibane renmei as well as Kinos journey as well as Voices of a Distant Star which is also known as Hoshi no koe
The Place Promised in Our Early Days as well as 5 Centimeters Per Second and all the .hack anime series also qualify too as well as barefoot gen all of them are great for this category of anime and/or manga
^I know that feel. I watched it three times in the span of four days (dont ask why) and I cried like a baby each time. I don't think I'll be able to watch it without crying and I don't even cry that much usually. xD
I also just finished a manga called Orange, and boy that had some sad parts too. I really reccommend it if someones looking for a good shoujo type of manga.