Might be reassuring but she’s probably only that way with you because she feels safe and secure enough to get upset. It also sounds like you’re feeling it more because you’re anticipating her going off to nursery and wanting this time to be special - which is totally fair!
As said above, when they are comfortable to cry and be miserable with you it shows they feel safe and loved enough to be the real them. It’s hard I know but grumpy phases happen and will pass!
I feel you, going through similar and I'm already back at work. Broke down this morning 😭 at work. Although seeing him smile when picking him tonight brightened my day slightly (first time he's smiled when I've collected him)