Lauren,
Kent gave you a good suggestion. Let me add a comment.
You are obviously young enough to go back again. Don't try to do everything. Be flexible. Want to stop somewhere and stay a day or two longer? Do it!. Better to skip areas and go back on another trip than to do one of those "If it is Tuesday, it must be Belgium".
So start where you think you would enjoy the most, and if you enjoy it so much, skip the back half and go back again. When we first traveled, we thought we'd never again go back to Europe. Thirteen trips later, we are planning another one possibly next year.
You can never spend too much time in a small place. Three weeks could do Italy. We did it in that time but skipped Venice, Lake Como and Milan as we had done them on a previous trip. We also visited relatives so that used a few days.
Have fun!