I believe that worry, guilt, and regret are pretty useless emotions.
Worrying doesn't fix anything, guilt doesn't help anything, and regret doesn't change anything.
I'd like to add another emotion to this list; hope.
Don't get me wrong. Hope is part of what fuels us as writers, and as human beings. We all want to see our books on the NYT list, have Oprah recommend us, have mile long signing lines and huge advances.
But like those other emotions, hope is a reaction rather than an action. We hope for things to happen when we don't have control over them, and then our hopes often get crushed. That leads to doubt, depression, and a bunch of other unhealthy emotions.
Consider a marathon runner. She trains, and practices, and conditions, and keeps at it until she can, indeed, run a marathon. There's no hope involved, only determination.
Now you could say that a marathon runner is in control of her future, because success doesn't involve other people. Writers need agents and publishers and sales reps, because they can't succeed without them.
That may be true, but we can still learn a few things from the marathon runner:
1. Set Goals Within Your Control. You have control over how many pages you write per week, how many submission you make per month, your marketing plan, and how you budget your professional time.
2. Understand Expectations. Whenever you do anything in publishing, you should have a clear idea of why you're doing it, and what results will be acceptable to you. Know what these results are before attempting anything, and you can never be disappointed.
3. Push Yourself. You don't know your limits until you go past them. Don't let anyone, including you, tell you what you can and can't do. You need to find out what you're capable of by doing it, not by guessing.
4. Learn From Your Mistakes. Actually, there's no such thing as a mistake if you learn from it. Evolution involves stress and failure--it's the only way to become stronger.
5. Know the Competition. You aren't competing with me, or Dan Brown, or the people your writer's group. The only competition you have is with yourself. If you work hard, you can always get better. Getting better is always good.
As I've said many times, success will ultimately come down to luck. But luck favors those prepared for it. You may not have control over the publishing world, but you do have control over how you act, and how you react. Don't waste your time hoping for success. Instead, like a marathon runner, you should be training for it.