There are usually too many things we would like to change in our past. Mistakes, misunderstandings, and of course, our thoughts. We would prefer that we were wiser back in that time, behaved more thoughtfully and with more care towards the people who deserved (and really needed) our attention and that we were less preoccupied with the things that actually were not so important.
Unfortunately, we cannot do anything about the past, but the changes that can improve our today and future life are more than possible. We can change attitude towards some things that spoil our good mood. You can be in your early twenties or your fifties, the only thing that truly matters is the extraordinary, almost magical power that you have: you can change your entire life by switching your thoughts and doing some mental work to make your life better without the negative impact of your worries.