"The silver bullet against regret is to make something valuable of the life you have now. It doesn't have to be valuable in the eyes of society -- big house, fancy job, Nobel Prize, etc. -- just valuable in your eyes.
For example: knowing you've been helpful to someone who really needed it; being with someone whose face lights up when you're around, or who makes your day brighter; being able to pursue a goal/hobby/passion that feels worthy to you, or merely fulfilling; relishing the freedom to try new things where otherwise you'd be tied down . . . the limit isn't your circumstances, but instead the perspective and imagination you bring to your circumstances.
Short version: When you like where you're going, you tend to look back a lot more fondly on what got you there.
Shorter version: Look forward, not back."