If you really could go back and choose differently the first thing that would happen is you would eventually become lazy. You wouldn't have to think too deeply or listen to your guts in making your choices because you could effectively just undo and retry.

Your life would basically revert to depth-first search instead of carving out your own, unique path in life. This would also mean it would be much, much easier to be unhappy because you could just switch to another branch and see if the grass is greened on the other side instead of having to learn to see the green you have now.

The inability to redo choices is like death: without death, life itself would be dead. It could not pressure you to make choices, to overcome yourself, dare to go for the love of your life or that enormously ambitious career, and eventually navigate yourself into a life where you grow to be happy with what you have instead of what you want. Without the clock ticking you wouldn't make the best of you in life. Similarly, if you could backtrack to your old choices you wouldn't be compelled to learn from them and they would be rendered worthless.