Most people were still trapped in the past. They cannot live very well because they always thought of what might could have been if they do this or do that. But the thing is this... if you do this or do that do you think you were very happy today? do you think if you do the things that you were supposed to do your life will be perfect?
If you hate your life now because you can't forget your ex-girlfriend, by the way the two of you separated because you're such a cheater and she found out the shit that you are doing. Now, you were always thinking about "what if I didn't cheat?", "only if I completely being honest with her, she will still be on my side". Do you think if you didn't cheat on her the both of you will still be together? Let's say you didn't cheat but what if she is the one who cheats? What if the two of you separated because of other reasons? You can't assume that things will go well if you did the right thing. Some ugly things will still happen even if you only do good things in life.
And one more thing, what you have now may not be yours if you do things differently in the past. For example, you have a car now. Do you think you can have it if you and your ex-girlfriend were still together? I am pretty sure you will spend a lot more money if the two of you were still together and that will stop you from buying a car.
If your job is really good now, do you think you will be in that position if you two are still together and spending too much time together? Different things may happen so stop dwelling with the past and appreciate your life completely now. Appreciate the present, open your eyes and you can see that there are still some good things that is happening in your life.
The problem with people is they cannot see the good side of their mistakes. Some people regret of having a child too early because they thought that they didn't enjoy too much time being young, they cannot party anymore, they cannot drink at late night anymore. They didn't think that if they do not have a child too early then they maybe have a miserable life because they were not responsible before. Having a child too early will make you responsible in life, it will make you strong, independent and more reliable. So never feel regretful again having child too early in your life because you were a better person now.
You always thought that correcting mistakes from the past will make your life better. The reality is your life is much better now because you made mistakes from the past. You already knew what is needed to do and you already knew what to do next. You are a better person now so be happy that you made mistakes from the past.
Granted that you can rollback time, do you think that you will not make the same mistakes again? do you think that you are completely in control because you already know what will happened? Definitely NOT. Even if you rollback the time you will still make mistakes along the way, you will still have problems so move on and don't waste your time thinking about the stupid past.