The problem when it comes to us making decisions is that we are not rational people and often tend to decide on things following some sort of persuasion, most likely on the spur of the moment. Even when we have given some thought to a decision that we are about to make we are persuaded one way or the other. People use all sorts of techniques to make us part with our time and money without us giving it too much thought.
Because we are tempted by persuasion we often make our decisions then and there and then regret what we have done, having bought something we didn't need and committing to do something we really didn't want to do. Once you have made the decision you are probably stuck with that decision. Some are big and last many years while other less important.
What we find is that we tend to regret decisions we didn't take, like not changing jobs, or not marrying the person you dated when you were younger. These decisions that we didn't make leave us with very many unknowns and enables our minds to make up all sorts of possibilities of what our lives would have been if we have chosen differently.
We seldom regret decisions that we have made, like taking that job or having a child at a particular time in our lives. Because we know the outcome that those decisions have given us, we are far less likely to regret them, and we often make the best of the decision once it has been made.
The way to make an important decision that will last, is not to fight over it in your mind, don't constantly struggle trying to consider which decision is right. As we have seen, you only tend to regret what you don't do and not what you do do.
When you have a difficult decision to make, think about it carefully, then take your mind right off it, by doing a crossword or a Sudoku puzzle or anything that would absorb your conscious mind. Your conscious mind doesn't have the capacity to really work on a big decision for too long, but your subconscious mind does, and by removing the thoughts of the decision from your mind you are giving your subconscious time to consider the decision.
Your subconscious mind is much better at making the right decision for you and as such will get the decision right every time, you just need to get the environment right for it to do the right thing.
In a nutshell, you only regret decisions that you have not made and cannot influence, so why regret. And to make the most important decisions, take your mind off it, and you will get the right decision everytime.