I don't know about "struggle," but I did learn the importance of classroom management and how critical it is to be consistent with its implementation.
Personal time managment and record keeping. I spent the first half of the year barely treading water. I was unprepared for all the nonteaching obligations I had so I took a lot home and was essentially giving up my nights and weekends planning (it gets better after your first year). Record keeping...only because I hated it :)