Can you change a bad decision made 20 years ago?

We all make bad decisions. I use to spend a lot of time thinking and wishing that I hadn't made some of the decisions that I made. Maybe you understand exactly what I mean. I wasn't full of regret, I just really wished that I could go back and reverse some decisions or actions. And yet, I was finally able to do it after figuring out how to do it.
We can't really go back in time. But we can effect the emotional outcome of a bad decision. A few years ago, I started thinking about a friend of mine in junior high school and high school. We were close friends, but we got involved with different groups of friends, and wound up treating each other like strangers. Whats more remarkable is that while we were close friends, things were going very well for me. After we were no longer friends, it created a lot of awkward accidental encounters. After 18 years, I tracked my friend down, and over lunch, I apologized for the way things were. I expressed how I felt, and how I regretted that we weren't friends. He said he felt the same way. I immediately felt relieved, and felt free of that bad decision.
Here's another example: I always regretted some of the decisions I made in high school about organizations I did not join, colleges I did not apply to, etc. I recently actually gave advice to some high school students (from my high school) to help them avoid the pitfalls of my own student career. It did wonders for me. The next day I woke up feeling that I actually never made those decisions. It felt great. By helping others, I helped myself.
The other way to do this, is to give yourself some slack. Accept that you're human, and don't be hard on yourself. The rest of the world is already hard on us, why ad to your own burdens. Sometimes you have to forgive yourself, and find a positive in the bad decision. There's always one.
The key is to find people that helped you along the way, that provided something positive, or that you were close to during the good times of your life. That's what you need more of. You don't really need to find anyone that was negative or was around you when things were not going well. I can't put my finger on it, but its a combination of the right people that we need to surround ourselves with in order for things to change and go in the right direction. Some people can just add to your good fortune, while others will subtract. We often don't know who's who.