Basically, that’s what to second-guess something means. It means you begin to think twice about it, to have doubts about it, to question whether there’s a better option, even to be critical of it ...

短语 “get over something” 的意思是 “从不愉快的事情中恢复过来”；“get over someone” 的意思则是 “忘记一个人”，因为和这个人之间可能曾经发生过不愉快的事情，它多用在描述人们在分手后，忘掉过往和对方的语境中。