I literally cried every day of first grade, every day of second grade, and then a big chunk of third grade, mostly all from missing my mom, which is wierd because we were also - like you describe - super close and there weren't any issues at home. I did however also have OCD and could get upset if i didn't get my nighttime routine blown kiss from her (it was more obsessive/compulsive and not emotional), so maybe that played into it more than i realized?
when i was 11 i started to get really upset at the idea that i knew i would lose my mom, it's like i could feel her slipping away from me, i even remember talking to her about this. it didn't make much sense at the time but then suddenly around 12 it was like a rubber band snapping and i just didn't care at all. i couldn't get enough space.
don't know if that helps, but reading your post reminded me so much about my own worries and fears growing up.
maybe it's a little extreme, but i can't help0 but wonder if a part of this that i experienced was a result of CIO trauma. i was a super sensitive child and i know for a fact that my mom did CIO with me when i was a baby. she thought what a lot of mothers did at the time, that it was somehow empowering me to self-soothe. come on! we don't expect an infant to read or write but we can expect them to do something extraordinarily difficult for most adults even and completely manage their own emotions! its absurd (sorry to rant, i know i'm preaching to the choir here ;-) ). we expect that just because a baby stops crying that they have magically grown this supernatural ability to soothe themselves instead of accessing it practically and realizing that the baby has probably just reached the pitiful conclusion that nobody cares for them once it becomes dark and they are completely alone and it is useless even to cry. being such a sensitive child i think that would have traumatized me as a baby and made it so i had indescribable emotional reactions to separation later.
I doubt any of that applies to your situation, just thought i would throw it out there...