There are so, so many choices in life. There are the tiny ones that we make every day- for example, what cereal to have for breakfast. There are the choices that will have an impact on other choices, like what cereal to buy in the first place. There are the bigger choices- what to do with your life, whether to stay, whether to go. So many choices.
There are lots of people who will talk about regret, and say that they wish they'd chosen differently. A while back, I thought about that and decided it was a waste of time. Firstly, the choices I've made in the past are, as the sentence suggests, in the past. There's nothing that can be done to change them, so there's no point spending time worrying or thinking about it. In addition to this- humans are complex creations. We do not make our choices lightly, even when we think we do. We are always looking for what feels right, and we act accordingly. Therefore, every choice has reason, and that reason is the deciding factor at the time.
Of course, that's not to say that, in not regretting, I am removing the past as quickly as it happens. I'm not. What I'm doing is looking at it in a different way, finding the lessons in everything I'm doing and using those lessons to become a wiser and stronger person. I have found this immensely useful. I would recommend letting go of regret to everyone and anyone.
Lastly- whenever you are angry or upset or whatever with someone, and the choices they have made, remember that they have done what they think is right. That does not make it right; but it's a reminder that they are trying, just as you are. There are choices we will never understand, made by both ourselves and others. All we can do is proceed.