If it's in the past, you let it go by learning from it, and making future decisions accordingly. Analyze why you wish you had done it, what appealed to you about it, and what stopped you from doing it. Find other places where you can do things for others, and seek out the opportunity to volunteer. It does not have to be the super popular places. Some places don't get many volunteers. Find the ones where you think you can fit in, and ask them how you can help. You'd be surprised by how happy they are to have you!
In all honesty, there are things I still regret not doing, 35 years ago. I still think about them. But, I think the lesson learned by missing out on those opportunities made me a better person, because I stopped looking for excuses to say "no," and started doing what I knew I should have been doing all along!
Best of luck!