You could have been bigger than what you are right now if you just did the right thing from the past.If you just lift more weights from the past, if you just did it consistently then you are stronger now, you are more competitive now and you can out muscle your opponents.
If you just eat the right food from the past and exercise regularly for at least 30-45 mins, was that too hard? Maybe you are one of the hottest person alive and your body will be healthier than you ever thought.
If you just practice the skill that you want to develop for at least 1 hour a day then maybe you are one of the best in your chosen field, you might be a guru or some kind of a beast. (that time will not take longer than the hours you spent watching nonsense TV shows)
If you just take time to understand your partner, please him/her a little bit, understand the situation then maybe your relationship will not be in jeopardy, you will avoid divorce and your family will be in a good position.
If you just stop smoking the moment you feel bad about your lungs then maybe you will live a little longer, you don't need to go to hospital to attend scary checkups. You don't need to listen to doctor's advice and believe all of his life threatening lessons.
It is not healthy to live life with regrets but sometimes we need to remember those things so we can avoid to do it again and make our lives better.