The fear is definitely worse than the withdrawal itself. The physical part is bad but you'll get better so soon. What's one week in comparison with your entire time you've been using? Maybe the first 3 days will be worst but this feeling of freedom and hope just ahead will provide you with the strength you'll need. Think positive, imagine all the great things you will be able to do once you're free and happy.
I have been addicted to opiates for so long and I have missed so much! But I did it and I believe you are going to be just fine. The quitting time itself is not as hard as the days ahead. So start thinking of all the things you can think about that will make you busy. Even if you don't feel happy right away remember this is all part of the process.
Exercises will help you with the endorfin, whenever you experience cravings just live a moment at a time and soon you'll realize the craving itself doesn't last too long. You'll be saving money, also will be looking better and healthier. Think of the things you like to do, other than drugs. Anything is possible. For me it was music, books, going to different places, and small pleasures like watching you best series, playing games. Travel if you can.
Of course there will be moments you'll be sad. I felt quite depressed for a while but managed to feel some happiness every here and then. People can be quite supportive and you'll be surprised of how much money you'll either be saving or not worrying about it all the time.
Some people feel safer with supporting groups, you can practice in here. Just remember that most of us have gone through all of these feelings.
Keep posting! You can do this, believe in yourself and be patient.
Take care,
Erik