Hey... So just my interpretation..
"Bound by the surprise of our glory days"
She feels trapped by the sudden appearance (and now absence) of the best days of her life aka her time with her ex...
Thats what I got out of the line at least...
This must be taken in context. The other lyrics are:
"You'd know how the time flies.
Only yesterday was the time of our lives.
We were born and raised in a summery haze.
Bound by the surprise of our glory days."
She's looking back on their time together - their time together seemed like it was just yesterday. Yesterday, their love was wondrous, beautiful, amazing. They were happy together - like they were in a 'summery haze'. They were surprised at how wondrous it all was, surprised by their 'glory days'.