It really isn't so much that you cross-threaded it, but in reality it is almost always a piece of debris that went on along with the nut. One more common thing that gets stuck is a small piece of the threads themselves, which makes for a real nice mess when you try to back it off and the piece gets stuck in the nut. It rips up the threads when enough force is used to back off the nut.

If you truly put them all on by hand, and spun them down snug by hand, then it isn't likely to be cross-threaded. I would say there is a better chance you have some debris stuck in the thread area which has caused it to bind. You could try some lubricating spray and gently work the nut back and forth to see if you make any gains. Beyond that, it is likely you will need to get it to your mechanic.

If you truly put them all on by hand, and spun them down snug by hand, then it isn't likely to be cross-threaded. I would say there is a better chance you have some debris stuck in the thread area which has caused it to bind. You could try some lubricating spray and gently work the nut back and forth to see if you make any gains. Beyond that, it is likely you will need to get it to your mechanic.