to sulk: be silent, morose, and bad-tempered out of annoyance or disappointment.
She was crying because she was heartbroken, so I don’t think sulking is the right terminology here.
And yes, the danger was always present that she might find herself in a situation where she was unable to return to Equestria in time. This was the risk she took, and the unexpected development that got her caught after all these centuries was Alt. Luna teaming up with Alt. Celestia, allowing her to get the jump on Celestia.