If you have missed an opportunity, rest and recover, but at some point you should stop dwelling on the past. You don't have to accept it as fair, but you have to switch focus to what you can do to move forward.
You can do anything, but not everything. Careful use of time lets you do more, but time remains a limited resource. Determination and discipline can create or invite new opportunities. Therefore, whenever you choose, consider the opportunities that each path offers, the ones you can defer, and the ones you will miss. As early as possible, take on the things you will regret not having done.
Decide quickly, but not in haste. Indecision equates to idleness, and that itself may lose you the opportunity. Learn from not just your own experience, but on the understanding of others.
When you have chosen, commit. An agile approach is to re-evaluate with new information the goal that you are chasing. This does not mean to keep switching targets and never producing anything useful. It means adapting the way you approach the same goal - assuming you chose an appropriate one.
Finally, take death as an ending and starting point.