You’ll get used to it! Get a good, windproof jacket - polarfleece alone will not cut it - along with a hat and gloves. If you’re going to spend a lot of time outdoors, woolen underwear is your friend. It will itch when you first put it on. The itching will stop.
Good boots and warm socks are important, too. Even just walking around town, you will find that few things can ruin your mood as quickly as wet, cold feet.
A couple other things, not about Germany specifically because I’ve never lived there, but about being an American and moving to Europe:
European washing machines heat their own water. This is more energy-efficient but it takes longer. If you put your wash in and come back a half-hour later, you will be disappointed. You’ll get used to it. Dryers also work slower because they dry at lower temperatures.
Yes, you will learn to sleep under a duvet. It will end up on the floor for the first week or so, but you’ll get the hang of it. Feet, you will find, are an important part of the body’s temperature regulation mechanism at night.
Hot breakfasts are rare, but from what I know, Germans eat decent-sized breakfasts. You’ll be able to find cold cereal, but the normal thing is bread with toppings.
Stores close earlier than you’re used to and may not open at all on Sundays. Find out the local laws and learn to live with them.
Don’t assume people are being rude to you because you’re new/an outsider/an American. Rules of politeness are different. You’ll be excused for not understanding them at first, but err on the side of caution.
Don’t panic.
Have a great year!